Presentity is an term used to describe an entity that provides presence information to help other entities, such as individuals, know whether it is acceptable to initiate a communications session, such as a phone call, a videoconference or an instant message session. The word is a combination of words "presence" and "entity" and was first introduced in RFC 2778.
Presentity usually refers to a human. In this case presence information describes availability and willingness of this human to communicate via set of communication services. For example users of an instant messaging service (such as ICQ or MSN Messenger) are presentities and their presence information is their user status (online, offline, away, etc.).
A presentity can also refer to a group of humans, for example a collection of customer service agents in a call center. This presentity may be considered available if there is at least one agent ready to accept a call.