Summary: Prefixes are key morphemes in English vocabulary that begin words. The English prefix *extra-*, which means "outside," appears in a fair number of English vocabulary words, such as *extra*, [*extra*]terrestrial, and [*extra*]ordinary. You can remember that the prefix *extra-* means "outside" from the adjective [*extra*]ordinary, which refers to something which is "outside" or beyond what is usually considered ordinary.
