Phase
Definition
The position of a sound wave within its cycle at a given point in time. When two identical signals combine out of phase, they cancel each other. When combined in phase, they reinforce. Phase relationships between microphones significantly affect the character and weight of recorded sound.
In Simple Terms
Where a sound wave is in its cycle at any given moment. When two mics capture the same sound slightly out of sync, the waves can cancel each other and your audio sounds thin or hollow. Check phase whenever you use multiple mics on one source.
In Practice
Two microphones placed on opposite sides of a snare drum require the bottom microphone's phase to be flipped 180°, otherwise the two signals partially cancel and the snare loses body and punch.