The most commonly known values of it are 78.2 for water and approximately one for air.  
It is a number without dimensions and can be expressed as "epsilon over epsilon sub zero."  For this reason it can also be referred to as relative permittivity and in the cgs system it is equal to permittivity.  It is measured as the ratio between the capacitance of two capacitors, in which one possesses a given material and the other does not.  The most commonly known values of it are 78.2 for water and approximately one for air.  FTP, name this constant that measures the ability of a material to resist the formation of an electrical field within it.
It is a number without dimensions and can be expressed as "epsilon over epsilon sub zero."  
dielectric constant (accept early "relative permittivity" and prompt on "permittivity")