For 10 points, name this property held by physical quantities with no imaginary component.
In the absence of circular polarization, Jones vectors are depicted with values with this property. In quantum mechanics, operators corresponding to observables must have eigenvalues with this property, meaning the operators must be self-adjoint and Hermitian. For an AC circuit, this property is held by the active power, but not the reactive power. This property is not possessed by probability amplitudes, but is held by probability densities as they are the square (*)  modulus of the amplitudes. For a quantity represented by a phasor, the component with this property equals the magnitude of the phasor times the cosine of phase. When reactance is zero, impedance has this property and equals resistance. For 10 points, name this property held by physical quantities with no imaginary component.
In the absence of circular polarization, Jones vectors are depicted with values with this property.
real  number [do NOT accept "positive" or "negative"]