That's too restrictive a definition. For instance, I've built coherent detection systems with offset frequencies as high as 8 GHz. (That one was a homodyne interferometer for detecting submicron tin droplets moving at up to 3 km/s [Mach 9].)
The basic definition of optical coherence is the ability to form fringes. The fringes don't have to stay still. I took Joe Goodman's statistical optics class long ago, and his book is an excellent read: .
Cheers
Phil Hobbs