Real Dictionary

trade in
phrasal verb: If you trade in an old car or appliance, you give it to the person you are buying a new one from so that you pay less.

trade in in American English
to give (a used article) as payment to be credited toward a purchase
to give (one's used automobile, etc.) as part of the purchase price of a new one

trade-in
A trade-in is an arrangement in which someone buys something such as a new car or washing machine at a reduced price by giving their old one, as well as money, in payment.

trade-in in American English
a used car, appliance, etc. given or taken as part payment in the purchase of a new one
adjective: of or pertaining to the valuation of goods used in a trade-in
noun: goods given in whole or, usually, part payment of a purchase

trade-in in British English
noun: a used article given in part payment for the purchase of a new article
verb: to give (a used article) as part payment for the purchase of a new article