It's not a very clear term, unless it's in the context and traditions of a particular school.
Terminology is often like that. In Korea, a reverse punch is thrown with the leading hand. A punch from the rear hand is called a straight punch. In America, a reverse punch is thrown with the rear hand. I.E. if you're in a stance with your left foot forward, the reverse punch is thrown with the right hand.
The term "reverse elbow" is not one I can recall ever using. We teach elbow strikes as being forward or backwards, upwards, downwards or sideways. So an upward forward elbow strike would be bringing the elbow up into the target, similar to an uppercut.