In a BDSM context, a collar is a device of any material placed around the neck of the submissive partner. The standard form of a collar is a black leather band around the neck and often has metal D-rings added to allow the attachment of a leash.
Collars may be used in role-playing games involving humiliation because they have connotations of control and pet-like status, especially when worn with a leash.
In BDSM relationships, the collar symbolizes submission or ownership. The partner wearing the collar belongs to the Dominant and is under their power. In many cases, the collar is essentially interchangeable with a wedding ring. There is often a collaring ceremony between the Dominant and submissive during which the collar is placed on the submissive and certain promises are made.
Because of the way D/s relationships are often viewed by society, submissives often have removable collars or "day collars." Day collars are generally necklaces or even bracelets. It serves as a reminder of the relationship and the promises exchanged.