Definition of mail-in in English:

mail-in

noun

US
  • The act of sending something in by post. Frequently attributive, especially designating ballots, surveys, etc., in which results are collected by post. Also concrete: an item intended to be sent in by post.

Origin

1960s; earliest use found in The Sunday Times. From mail + in.

Pronunciation

mail-in

/ˈmeɪlɪn/