Longman Dictionary of Contemporary English
Wiktionary
a. Of a selection, the property of having to choose explicitly to avoid or forbid something; a decision having the default option being inclusion or permission; used particularly with regard to mailing lists and advertisement. n. 1 the act of opting out of something 2 (context British English) a school or hospital that has withdrawn from local control
Wikipedia
The term opt-out refers to several methods by which individuals can avoid receiving unsolicited product or service information. This ability is usually associated with direct marketing campaigns such as, e-mail ma rketing, or direct mail. A list of those who have opted out is called a Robinson list.
In the UK, the Mailing Preference Service composes the Robinson list.
In politics, an opt-out is when one level of government can decline or refuse to be part of a program designed by another level of government.
Usage examples of "opt-out".
Arizona's first Libertarian governor, Durwood Thornley, is proposing to extend the school voucher system to a whole variety of taxpayer opt-outs, and his critics are not very happy about it.