A prefix's an affix which's placed before the stem of a word. Particularly in the study of Semitic languages, a prefix's called a preformative, as they can alter the form of the words to which they're fixed. Examples of prefixes: unhappy : un's a negative or antonymic prefix. prefix, preview : pre's a prefix, with the sense of "before" redo, review : re's a prefix meaning "again". The word "prefix"'s itself made up of the stem fix (meaning "attach", in this case), and the prefix pre- (meanin… (More on Prefixes)