The term phase is used to refer to the part of a cycle that an oscillating system is in at a particular moment. For two sine waves of the same frequency that are not in step, one wave lags or leads the other in time. We can express the amount by which they are out of step as a phase difference. Usually phase difference is expressed as a fraction of a cycle or as a certain number of degrees (one complete cycle corresponding to 360°).
If two (or more) sine waves are completely out of phase (a phase difference of 180° or odd multiples thereof), there is complete cancellation when the waves are combined by adding. If the sine waves are completely in phase (a phase difference of 0°, 360° or multiples thereof), there is complete reinforcement. When the phase difference lies between these extremes, there is partial reinforcement or partial cancellation. The result, however, is always a sine wave of the same frequency as the ones being combined.