(1) type of program that assumes inclusion unless stated otherwise. (2) to remove oneself from an opt-out program.
The term opt-out is often used in relation to ad tracking programs. By default, ad tracking programs track everyone. In response to growing privacy concerns, some major advertising networks have offered individuals the option to be removed from tracking.
Opt-out is sometimes used to refer to email which assumes inclusion; sometimes having “Yes” automatically checked on a signup form, sometimes by adding your name directly to a list. (Although in the latter case the term email spam is often used.)
The terms opt-out and unsubscribe have subtly different, yet related, meanings. You unsubscribe to something to which you had previously subscribed. You may opt-out of something you never joined in the first place.
Opt-This, Opt-That: Putting Permission to Rest
Small Business Computing (October 18, 2002)