Leading Phase:
Assume that there are two sine waves, wave X & wave Y, with the same frequencies. When wave X starts a fraction of a cycle previous than wave Y, then wave X is said to be leading wave Y in phase. For this to be true, X should start its cycle less than 180° before Y. Figure below shows wave X leading wave Y by 90°. The dissimilarity can be anything mush greater than 0°, up to but not involving 180°.
Figure: Wave X leads wave Y by 90°.
Leading phase is sometimes expressed as a phase angle Φ such that 0°< Φ < +1800. In radians, this is 0°< Φ < +π. whenever we say that wave X has a phase of +π/2 relative to wave Y, we mean that wave X leads the wave Y by π/2 radian.