Leading phase:

The two waves can differ in the phase by any amount from 0 degrees, through 180 °, to 360 °.

Figure--   Two sine waves in phase opposition.

Assume that there are 2 sine waves, wave X and wave Y, with identical frequency. If wave X begins a fraction of the cycle much earlier than wave Y, then wave X is said to be leading wave Y in phase. For this to be true, X should begin its cycle less than 180 ° before Y. The figure shows wave X leading wave Y by 90 °of phase. The difference could be anything greater than 0 degrees, up to 180 degrees.

Figure   Wave X leads wave Y by 90 degrees.

Note that if wave X is leading wave Y, then wave X is somewhat to the left of wave Y. In a time line, left is earlier and right is later.