Phase

Two sound waves are 'in phase' when their compression and rarefaction half-cycles coincide exactly in time and space.
If two in-phase waves are added together they produce another wave of the same frequency but twice the amplitude.
Signals are 'out of phase' when the positive half-cycle of one coincides with the negative half-cycle of the other.
If two out-of-phase signals are added together they will cancel each other out.
Most real-world examples are caused when two waves of the same frequency are partially out of phase with each other, which results in partial addition or cancellation.
Phase differences can be caused by a time delay between the waves; this can be a function of distance.
Phase is quoted in degrees. This relates to the degrees of a sine wave.