1. A word that means its opposite.

An example is the word fast, which when referring to a fast runner means a runner who runs rapidly or swiftly; but when it refers to a fast color, it means a color that doesn't run at all.

Based on information from Word and Phrase Origins

2. A word which is the opposite in meaning of another word with the same spelling: Example of autantonyms include: cleave which means "to cling" or "to split"; "to dust" can mean to remove dust when cleaning one's house or to add dust as when putting powdered sugar on a cake.