*In computing Presence is defined as the "availability
and willingness of the user (presentity) for communication."
Presence information is published by individuals to other
systems users, known as 'watchers' or 'subscribers', to indicate
their communication state. Although not limited to IP communications
it has become synonymous with IP applications such as VoIP and
Instant Messaging. en.wikipedia.org/wiki/Presence_(IP_communication)