What is "extra-"

Extra- \Ex"tra-\ [L., fr. exter. See Exterior.] A Latin preposition, denoting beyond, outside of; -- often used in composition as a prefix signifying outside of, beyond, besides, or in addition to what is denoted by the word to which it is prefixed.

word-forming element meaning "outside; beyond the scope of; in addition to what is usual or expected," in classical Latin recorded only in extraordinarius, but more used in Medieval Latin and modern formations; it represents Latin extra (adv.) "on the outside, without, except," the old fem. ablative singular of exterus "outward, outside," comparative of ex "out of" (see ex-).

pre. outside, beyond.