Does anyone know the term for a pair of words/concepts in which one can have no meaning without the other?
For example 'hot' has no meaning unless contrasted against 'cold'.
They're called gradable opposites.
... hot and cold, which describe opposite ends of the scale of TEMPERATURE. Hot and cold are both gradable; for example, we can say "A is hotter than B," "C is fairly cold," "D is very hot," and so on. Between the opposite poles named by hot and cold, there is a mid interval, so that if something is neither hot nor cold, it might be warm, cool, or lukewarm, etc.
See also Opposite (semantics) - Wikipedia, the free encyclopedia