I am using the word opposite in two ways:
1) To refer to something that has an opposite; 'heat' is an opposite because it has an opposite, 'cold', whereas 'three' is not an opposite because there is no 'opposite' of three.
2) To refer to things that are opposites; to describe the relationship between opposites; 'heat' and 'cold' are opposites.
The first one (I hope) is a noun. Is the second usage as an adjective? I would think so as it describes a relationship, but one could say "the opposites heat and cold", in which case as a pair they are opposites, so would that just make them both 'an opposite', as used in the first definition?