Epithet is a word that can be used for either adjectives or nouns, as long as they are used like adjectives to qualify a noun or noun phrase. This is because nouns can be used to describe - as if they were adjectives. We say 'History lessons', not '
historical lessons', when we want to describe the sort of lessons that we are having; or use 'IBM' (a noun, the name of a company) to describe the product it makes - 'an IBM computer'. This is behaviour typical of an adjective. To avoid argument, it is convenient to use the word epithet (or its adjectival form epithetical) to label 'words that are [temporarily] being used to describe nouns'.
(The opposite - a word that names a thing while being in itself either a noun or an adjective - can be called a substantive.)