“Coherency” is the normalized cross-spectrum at a specific frequency, i.e. for two signals A and B it is defined as Cab/sqrt(Pa * Pb) or equivalent Cab/sqrt(Caa * Cbb). Coherency is a complex-valued number with a magnitude (or “modulus”) smaller than or equal to 1 and an angle corresponding to the phase lag between the signals.

The modulus of the coherency is known as the “coherence”. The coherence is a real-valued number between 0 and 1. Note that it is also not uncommon in scientific publications to report the magnitude-squared coherence.