A stationary wave is that with all points in the medium are always in phase with each other, at any point of time.
Any transverse wave or a wave has a constant amplitude at each point in the medium. Usually the amplitude is same at all points. But, all the points are in different phases at any one point of time. Those points in phase are at a distance of wavelength from each other.
In a stationary wave all points are at a phase 0, at t =0, at phase π/2 at T/4 time, at phase π at time T/2 ...
Stationary waves are a result of interference of two waves of same frequency in opposite direction. This happens on a guitar string which is tied on two ends and is plucked aside in the middle.