what are Coherent Sources

The two sources of light, whose frequencies (or wavelength λ ) are
same and the phase difference between the waves emitted by which remains constant with
respect to time are defined as coherent sources.
They are obtained from the same single source.
Laser light is highly coherent and monochromatic.
The distance between them is small.
These can be apparent or real.
Their state of polarization is the same.