Blanket consent is a mutual agreement that consent will be voluntarily waived. In other words, comprehensive consent is given, ahead of time, for all acts and situations that will arise in the future. Once blanket consent is given, it is generally not revoked.
Blanket consent, as it applies to sexual relationships, is generally used in a BDSM context. It was originally used only in committed relationships, but in recent years has been controversially applied to play sessions as well. Some submissives will wear a collar to show their blanket consent.
Blanket consent is sometimes called consensual non-consent and metaconsent.