The forbidden fruit was an apple

The Hebrew text of Genesis 3 says only peri ("fruit"), with no species given. "Apple" enters the Western imagination through medieval European art and literature, most plausibly from a Latin pun: malum means both "apple" and "evil/bad" (as an adjective), so a Latin-speaking culture reading the Vulgate's malum arborem ("evil tree") had an obvious visual joke sitting right there in the vocabulary.

Outside that European tradition, guesses vary by region: Jewish and Middle Eastern commentators have proposed the fig (Adam and Eve cover themselves with fig leaves moments later, in the same chapter), the grape, the etrog (citron), or wheat. None of these is stated either; they're all inferences layered onto a text that deliberately doesn't specify, which may be the more interesting fact than any single guess.