Phase velocity $$v_{\text p}$$ of any point A on the wave, is the velocity with which this point propagates from A to B. Also drawn in is the wavelength $$\lambda$$. This is here the distance between A and B. The phase velocity is given by the distance per time, so in this case wavelength $$\lambda$$ per period $$T$$:$v_{\text p} ~=~ \frac{\lambda}{T}$