What Defines Coherent Sources?

[Amith2006]

Sir,
Coherent sources are defined as sources having same frequency and constant phase difference. In some books it is given that coherent sources have constant phase difference only. Which is correct?

 

[Hootenanny]

If two waves had different frequencies would they have a constant phase difference?

~H

 

[arunbg]

Constant phase difference will do as this in effect would mean that the sources possesses the same frequency.
But the reverse is not applicable.
Frequency can remain same with varying phase difference over time .