Someone who subscribes or “opts-in” to your mailing list, as used in email marketing. Also called subscriber.
May be used alternately as a noun referring to one or more subscribers to your list (opt-in, opt-ins or optin, etc), or as a verb (opt) denoting the act of subscribing (opts in, opts into, opting in, etc).
The use of a hyphen in the term is optional, and omitted almost as frequently as used.