What is it called when a word can serve more than one part of speech? 'Aggregate', for example, can be used as a Noun, an Adjective, or a Verb.
These are termed homonyms (to be more exact grammatical homonyms).
In linguistics, homonyms, broadly defined, are words which sound alike or are spelled alike, but have different meanings.
A more restrictive definition sees homonyms as words that are simultaneously homographs (words that share the same spelling, regardless of their pronunciation) and homophones (words that share the same pronunciation, regardless of their spelling) – that is to say they have identical pronunciation and spelling, whilst maintaining different meanings.
The relationship between a set of homonyms is called homonymy.
Examples of homonyms are the pair stalk (part of a plant) and stalk (follow/harass a person) and the pair left (past tense of leave) and left (opposite of right).
A distinction is sometimes made between true homonyms, which are unrelated in origin, such as skate (glide on ice) and skate (the fish), and polysemous homonyms, or polysemes, which have a shared origin, such as mouth (of a river) and mouth (of an animal).