Phase describes the stage that a particle has reached in a cycle. When an object vibrates, the movement from its initial position to a point of maximum displacement in one direction, followed by a movement in the opposite direction and a return to its initial position is called one cycle.
Particles are in phase when they are at the same stage in the cycle at the same time, and out of phase when they are at opposite stages. Phase is measured by the angle of rotation, in degrees or radians.
The rate of change of phase is termed frequency. (Source - reduced from Barry Truax, Handbook for Acoustic Ecology CD-ROM Edition, Cambridge Street Publishing, 1999 - CSR-CDR 9901)
See also:Beating, Frequency, Harmonic, Oscillation, Phase-shifting, Phasing