Coherent source
Definition
Coherent sources are wave sources that march in lockstep — same frequency and a fixed phase relationship — so their interference pattern stays stable instead of flickering.
Technical definition
Two sources are coherent if their phase difference Δφ is constant in time. Only coherent waves produce a stationary interference pattern; lasers are coherent, ordinary bulbs are not.
Example
In Young's double-slit experiment, a single slit first makes the light coherent before it hits the two slits — without that step the bright and dark fringes would wash out.
Coherence is not the same as being monochromatic. Two identical tuning forks are nearly monochromatic but still drift out of phase; true coherence needs the phase relationship locked, which is why lasers beat light bulbs for holography.