In general, these are called homonyms, especially in cases like fast (period of not eating) vs fast (moving quickly), where neither is derived from the other. In cases like these, they seem to be stored as separate lexemes in the brain, and it's just coincidence that they look the same. This can even happen when the words are related, if they've diverged enough: even though fast (moving quickly) comes from fast (securely, as in "stuck fast"), modern English-speakers see them as separate words.
Zero derivation (and derivation in general) usually refers to a different process, where there's only one underlying lexeme in your mental model of the language, and the part of speech is determined by how you use it (or what you attach to it). For example, if I say "the new light bulbs really greenified the whole room", you don't have a mental lexeme for "greenify" (especially since I just made it up)—instead, you have lexemes for "green" and "-ify" that you stick together to get the meaning.