Extra definition: beyond or more than what is usual, expected, or necessary; additional.

extra 1 of 4 adjective ex·​tra ˈek-strə Synonyms of extra 1 a : more than is due, usual, or necessary : additional

Extra implies excess, surplus, something additional beyond what is expected or necessary. In Latin, extra means “outside” or “beyond” and it’s that meaning we find when it prefixes words like extraterrestrial (a creature originating from “outside” planet Earth) or extrajudicial (describing something “beyond” what is allowed by a court).

Typically, we think of "extra" as being a modifier for other words: It can be used adjectivally to indicate an increased amount of something (as in, "I made an extra $20 this week") or an adverb to indicate that something is more of whatever it is than usual ("Wow, that Smurf is extra blue").