Polysemy: the existence of several meanings in a single word.
Word Origin: from New Latin polysēmia, from Greek polusēmos having many meanings, from poly- + sēma a sign.
Not to be confused with homonymy. From Quora:
There's a good deal of overlap in meaning, but the key difference is that a polysemous word is one word with different meanings, while homonymous words are different words that share a shape (usually both spelling and pronunciation). For example, most people would consider the noun tire (the things on your car) and the verb tire (what happens when you exercise) to be completely different words, even though they look and sound the same. They're homonyms. On the other hand, most people agree that there is only one word offense, but that it has various meanings which are all related: the attacking team, a criminal act, a feeling of being insulted, etc. So offense is polysemous.