SIP
(1) Short for Session Initiation Protocol, it is an application-layer control protocol; a signaling protocol for Internet Telephony. SIP can establish sessions for features such as audio/videoconferencing, interactive gaming, and call forwarding to be deployed over IP networks, thus enabling service providers to integrate basic IP telephony services with Web, e-mail, and chat services. In addition to user authentication, redirect and registration services, SIP Server supports traditional telephony features such as personal mobility, time-of-day routing and call forwarding based on the geographical location of the person being called.
See "The Difference Between VoIP and PSTN Systems" in the Did You Know... section of Webopedia.
(2) Short for single in-line package, a type of housing for electronic components in which the connecting pins protrude from one side. Compare with DIP and PGA.
A SIP is also called a Single In-line Pin Package (SIPP).


