Two sources are said to be coherent if they emit light waves of same frequency, nearly equal amplitude and are always in phase with each other.
Realization in practice
In practice it is not possible to have two independent sources which are coherent. But for experimental purpose, two virtual sources formed by a single source can act as coherent sources. Methods have been devised where (i) interference of light takes place between waves from the real source and a virtual source (ii) Interference of light takes place between waves from two sources formed due to a single source.