additional; beyond what is usual, expected or arranged
accessory; excess
Cases of word usage
1) a payment in addition to what is usual, necessary or expected, e.g. and extra payment to workers (bonus)
2) a person, who is fond of making jokes; an extra playing-cards with the highest value, used in some games (joker)
3) an extra advantage, benefit or product from a large activity or process (spin-off)
4) something added later to improve or complete; an extra and separate addition to a newspaper or magazine (supplement)