Model Release
A signed agreement in which a recognisable person consents to the commercial use of their image.
In depth
Editorial and artistic use generally does not require a release, but advertising and endorsement almost always do, and stock libraries will not accept recognisable people without one. Releases should be linked to the specific frames they cover, which is usually done by recording the release identifier in the image metadata.
Examples in practice
A stock agency rejects an otherwise strong street portrait because no release accompanies the recognisable subject.