COHERENCE :

1) Visible light energy is emitted when the excited electrons in atoms undergo transitions to the ground state.

2) In ordinary light sources, these transitions take place at random in time and the light waves received at a point on a screen bear no definite phase relation among them.

3) But, in a laser source, electronic transitions take place in an 'orderly way' and the light waves emitted have a consistent phase relation which does not change with time.

4) This is called 'temporal coherence' and is the most important characteristic of laser light.