phase 
A position, or point in time, on a waveform. The basic sine wave, such as an analog signal, is a series of undulating curves that peak and dip. In its simplest form, the wave starts at the zero point on an imaginary horizontal line, gradually climbs to a peak a bit further down the line, dips down past the line to a valley, and finally returns to the line. You would shift the wave's phase by moving the entire wave backward or forward from its point of origin on the line.