Praseodymium is a soft, white metal and a member of the lanthanide group of elements.
Practical Number
A positive integer m such that every natural number n not exceeding m is a sum of distinct divisors of m.
Prandtl Number
A dimensionless group often used in heat exchange calculations.
A soft, white metal and a member of the lanthanide group of elements.
Pre Combustion Chamber
A chamber in the cylinder head of some engines into which the fuel is injected, ignited, and partly burned, the rest of the fuel being thrown out into the main combustion space where combustion is completed.
Pre Ignition
Ignition of the mixture of fuel and air in the combustion chamber before the passage of the spark.
Pre Triggering
The technique used on a DAQ board to keep a continuous buffer filled with data, so that when the trigger conditions are met, the sample includes the data leading up to the trigger condition.
An amplifier that raises the output of a low-level source for further processing without appreciable degradation of the signal-to-noise ratio.
Related to the period before life appears on a planet.
The geological era covering about 4.6 billion years ago to 570 million years ago.
Precast Concrete
A concrete horizontal structural member that is cast and cured in other than its final position.
Precedence Effect
Delayed sounds are integrated by the auditory system if they arrive at the ear within 20 to 40 msec of the direct sound.
A modern term, derived from the precession of the equinoxes and meaning a motion around a cone of the rotation axis of a spinning body.
Precession Vector
In a gyro, a vector representing the angular change of the spin axis when torque is applied.
A solid that appears in the form of fine particles in what was previously a clear solution because of a chemical reaction.
The conversion of a dissolved substance into insoluble form by chemical or physical means.
Precipitation Hardening
Increase the hardness of a supersaturated solid solution by heat treating it to cause a second phase to precipitate out.
Precipitation Heat Treatment
Artificial aging in which a constituent precipitates from a supersaturated solid solution.
Precision Casting
The full or partial setting of a resin or adhesive before the clamping operation is complete or before pressure is applied.
Preferred Noise Criteria
Introduced in 1971 to correct some of the early problems with the levels set for Noise Criteria curves.
Preliminary Analysis
This is a hazard identification and frequency analysis technique that can be used to assess potential hazards at an early design stage.
Gradual and biologically normal loss of acute hearing with advancing age.
Defined as the force exerted per unit area.
Pressure Altimeter
An altimeter that utilizes the change of atmospheric pressure with height to measure altitude.
Pressure Bag Moulding
A moulding technique in which a flexible bag is placed over the contact layup in the mould, sealed, and clamped in place, and pressure applied by compressed air, which forces the bag against the part while the part cures.
Pressure Coefficient
A dimensionless value which acts as a means of indicating the local pressure at some point of interest around a body, and which is independent of velocity.
Pressure Drop
A measure of the resistance the gas stream encounters as it flows through an air control device or other piece of equipment.
Pressure Gauge
Instrument for measuring pressure.
Pressure Gradient
The change in pressure per unit of distance.
Pressure Head
Pressure head is one of the main characteristics of a pump.
Pressure Relief Valve
A type of valve used to control or limit the pressure in a system or vessel.
Pressure Switch
An electric switch operated by fluid pressure.
Pressure Type Anemometer
Any instrument that measures the wind speed by measuring the pressure exerted by the wind upon the sensor element of that instrument.
Pressure Welding
Pressure Zone
As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.
Containing air, or other gas, at a pressure higher than ambient.
Pre-Stressed Concrete
A method of construction whereby compression forces are applied to concrete elements with the goal of counteracting the tension that would otherwise occur due to loads.
Preventive Maintenance
Visual, mechanical, electrical, and electronic checks that are made to determine whether or not equipment is functioning properly.
Prick Punch
Primal Glow
Believed to be the cosmologically redshifted radiation released by the Big Bang itself.
Primary Bond
Interatomic bonds that are relatively strong and for which bonding energies are relatively large.
Primary Cell
A cell or battery which is not intended to be recharged and is discarded when the cell or battery has delivered all its electrical energy.
Primary Energy Source
An energy source that occurs naturally eg fossil fuels, biofuels, wind energy, solar.
Primary Reference Temperatures
A number of boiling and freezing points are used to define the thermodynamic temperature scale.
Primary Standard
A stable, high-purity material used in titrations and other chemical analyses to prepare solutions of precisely known concentration.
Primary Winding
The winding of a transformer connected to the electrical source.
Prime Numbers
A prime number has only two different factors. They are 1 and the number itself. 13 is a prime number because the only factors of 13 are 1 and 13. Note that 1 is not a prime number.
Prime Verical Circle
Passes through the east and west points of the horizon.
Primitive Pythagorean Triangle
A right triangle whose sides are relatively prime integers.
Primitive Root of Unity
The complex number z is a primitive nth root of unity if zn=1 but zk is not equal to 1 for any positive integer k less than n.
Prince′s Metal
An alloy resembling brass, consisting of three parts of copper to one of zinc.

Principal Component Analysis
Constructing new features which are the principal components of a data set.
Principal Curve
A nonlinear principal component axis.
Principal Quantum Number
The number related to the amount of energy an electron has and therefore describing which shell the electron is in.
Principal Vertical Circle
Passes through the north and south points of the horizon and coincides with the celestial meridian.
Principle of Uniformity
The principle that the laws of physics here and now are not different from the laws of physics in previous or future epochs of time, or elsewhere in the Universe.
Printed Circuit Board
A flat, insulating surface upon which printed wiring and miniaturized components are connected in a predetermined design and attached to a common base.
A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or through which it is refracted.
Prismatic Battery
A cased rectangular cuboid battery cell with a hard case that may or may not need additional structural support.
Privacy Index
A measure for rating the speech privacy performance of an architectural space.
A number between 0 and 1 which represents how likely an event is to occur.
A device used to send back information.
Process Capability
A measurable property of a process to the specification.
Process Engineered Fuel
A processed solid fuel, made from segregated plastic and paper, derived from industrial, commercial, and residential sources, for use by utilities and industry.
A substance that is produced during a chemical change.
The gear tooth profile is the line which defines the tooth shape in a cross-section of the gear perpendicular tothe plane of rotation.
Profile Drag
A list of instructions that tell a computer how to do something.
Program Counter
A microprocessor register that holds the memory address of the next instruction to be fetched.
Programmable Logic Array
A programmable logic device in which both the AND & OR arrays are programmable.
Programmable Logic Device
Generic name for an integrated circuit offering a vast array of logic function building blocks that the circuit designer programs to interconnect for specific applications.
Programming Languages
A list of programming languages with their main fields of use.
Projectile Motion
A type of motion that occurs near the surface of the Earth when the only force acting on the object is that of gravity.
A unicellular organism with a single chromosome, no nuclear envelope, and no membrane-bounded organelles.
Prokaryotic Cell
A prokaryotic cell has no nucleus and no cellular organelles.
Prolate Spheroid
An ellipsoid of revolution, the longer axis of which is the axis of revolution.
An acronym for programmable read only memory.
A radioactive element of the rare earth series.
Prompt Neutrons
Those neutrons released coincident with the fission process, as opposed to the neutrons subsequently released.
Pronic Number
A number of the form n(n+1).
Unit of power.
Proof Stress
Stress that will cause a specified permanent deformation.
Waves traveling through a medium.
Propagation Anomaly
A change in propagation characteristics due to a discontinuity or abnormality in the medium of propagation.
Propagation Loss
The attenuation of signals passing between two points in a transmission path.
Propagation Path
The route by which a signal, or a portion of a signal, travels from transmitting antenna to receiving antenna. Possible paths include the direct path, reflected path, skywave path, refracted path, etc.
Propagation Test
Test to look at how an event such as combustion propagates from one component to the next.


A member of the alkane series, a colourless, odourless gas at room temperature. In liquid form it is often referred to as Liquid Petroleum Gas (LPG).



Used as an inexpensive alcohol. Also known as propyl alcohol.

Alternative name for Acetone.
Propargyl Alcohol


The simplest stable alcohol containing an alkyne functional group.

An assembly of radially disposed blades with an airfoil shape that when rotated in air produce thrust.


A colourless highly flammable gas.

Propene Acid
Alternative name for Acrylic Acid.
Propenoic Acid
Alternative name for Acrylic Acid.
Proper Divisor
The integer d is a proper divisor of the integer n if 0
Proper Fractions
A fraction whose numerator is of lower absolute value than it's denominator.
Proper Time
The time interval measured by an observer in his own reference frame.
A statement of equality between two ratios.
Proportional Limit
The point on a stress-strain curve at which the straight line proportionality between stress and strain ceases.
A relationship in which a quantity is related to the second quantity.
A driveshaft on a rear wheel drive vehicle that transfer rotational power from the engine-gearbox at the front to the differential at the rear.
Propyl Alcohol
Alternative name for Propanol.
Alternative name for Propene.
Propylene Glycol


A colourless, nearly odorless, clear, viscous liquid with a faintly sweet taste, hygroscopic and miscible with water, acetone, and chloroform.

A radioactive element.
Protective Devices
Devices, such as fuses, diodes and current limiters which interrupt the current flow, block the current flow in one direction or limit the current flow in an electrical circuit.
Protective Relay
A relay, the principal function of which is to protect service from interruption, or to prevent or limit damage to apparatus.
A very large, naturally occurring polyamide formed from a selection of the 20 or so naturally occurring amino acids.
Protein Kinases
Enzymes that phosphorylate certain amino acid residues (most often Ser, Thr, or Tyr) in specific proteins.
Protein Targetting
The process by which newly synthesized proteins are sorted and transported to their proper locations in the cell.
The geological period from 1.0 billion to 570 million years ago.
The geological eon covering about 2.6 billion years ago to 570 million years ago.
A diverse group of eukaryotic microorganisms.
The lightest element of hydrogen of unit mass and the most prevalent in the universe.

The set of conventions that govern the treatment and formatting of data in an electronic communications system.
The central nucleus of an atome is formed from protons and neutrons. The nucleus of a hydrogen atom has a single proton.
Proton Acceptor
The acceptor of a proton in an acid-base reaction.
Proton Donor
Because a free H+ ion is technically a bare proton, acids are sometimes referred to as proton donors because they release hydrogen ions in solution.
Proton Exchange Membrane Fuel Cell
Works with a polymer electrolyte in the form of a thin permeable sheet. This membrane is small and light and works at low temperatures around 80deg;C.
Proton-Motive Force
The electrochemical potential inherent in a trans-membrane H+ concentration gradient.
Proton-Proton Chain
In the Sun and other less massive stars, this chain is the primary source of heat and radiation.
Any of the sun′s planets as it emerged or existed in the formative period of the solar system.
Protoplanetary Disc
The disc of dust surrounding a star out of which planets might form.
In the philosophy of science, a protoscience is any new area of scientific endeavor in the process of becoming established.
The sun as it emerged in the formation of the solar system.
A tool used to measure angles.
Prussic Acid
A solution of Hydrogen Cyanide in water.