Initial Phase

The initial phase $\phi$ (or phase offset), in radians, gives the position of the waveform cycle at $t=0$.

Sine function with $\phi = 0$ ( $\textcolor{blue}{\sin(\omega_0 t + 0)}$):

Sine function with $\phi = \pi /2$ ( $\textcolor{blue}{\sin(\omega_0 t + \pi/2)}$):

... or equivalently $\cos(\omega_0t + 0)$.