The difference in phase between two sinusoidally varying quantities. The displacement x1 of one quantity at time t is given by x1 = asinωt, where ω is the angular frequency and a is the amplitude. The displacement x2 of a similar wave that reaches the end of its period T, a fraction β of the period before the first is said to lead the first quantity by a time βT; if it reaches the end of its period, a fraction β of the period after the first quantity it lags by a time βT. The value of x2 is then given by x2 = asin(ωt + φ). φ is called the phase angle and it is equal to 2πβ.