I've read in an answer to this question, of which I think that it was a very good, but somewhat long one, (I include the context in which it was written):
Let me expand this point. When this happens, not necessarily for a light wave but for whichever sort of wave, we say that the medium is non-dispersive. This is what your teacher meant with his "principle of wave constancy". It is a property of the medium and does not generally hold true, for elastic waves, for sound waves (a kind of elastic waves), etc. It may hold approximately - this happens for sound waves in air - or not hold at all: think of gravity waves we see at sea surface or on a lake, on a pond...
I'm a bit confused about what it's meant by that. Normally one would think that gravity waves are propagating distortions of space, though they are actually called gravitational waves. I can imagine that the pull of gravity causes the water waves, or is this exactly what is meant by calling these water waves gravity waves? Or is it that the waves are called gravity waves instead of gravitational waves so these waves have to be interpreted in a different way? But again, in what way? I have a gut feeling about what is meant but can't find the words for it.
I could have looked it up, I guess, but in this way also other users can benefit to improve their knowledge.