A contronym is a word with more than one meaning (sense), as one meaning is the reverse (opposite) of the other. For example, the word cleave can mean "to cut apart" or "to bind together". In a sentence its meaning can be clear, otherwise the word itself is ambiguous. Contronyms are also known as auto-antonyms.
Many other words are polysemic, because they have more than one meaning. However, most polysemic words are not contronyms.