Phase difference ($\Delta \varphi ={\varphi }_2-{\varphi }_1$)

The difference of two wave phases at the same place and time, considered modulo $2\pi$. It determines whether two same-frequency oscillations reinforce, cancel, or are in phase quadrature.

For an oscillation written $A\cos (\omega t-\varphi )$ relative to $A\cos (\omega t)$, a positive $\varphi$ represents delayed maxima. The corresponding unwrapped delay is $\varphi /\omega$, while a single periodic observation determines the phase difference only modulo $2\pi$.