When two waves of same frequency and almost of same intensity (same amplitude) propagate in same direction, the intensity of resultant beam alternatively becomes maximum and minimum at different points in space. This phenomenon is called interference. 

All those points at which intensity of resultant wave is maximum are called positions of constructive interference or maximum. All those points at which intensity of resultant wave is minimum are called positions of destructive interference or minimum.

            When interference occurs the maximum and minimum are obtained at different points.