Acronym for Session Initiation Protocol. An IETF signaling protocol to establish multimedia sessions. Used for VoIP, but also for a growing number of related applications and services, e.g. video, instant messaging, presence and so forth. Although SIP is transport protocol agnostic by design, existing applications use either UDP or TCP (the latter sometimes referred to as TSIP).
SIP does not provide for security. See SIPS for an encrypted variant.