“Session Initiation Protocol Trunks”
Trunks are a telephony term relating to dedicated line from a service provider. SIP Trunks/SIP Trunking is the adaptation of the customary telephony trunk concept to the world of IP. SIP Trunking is a term used to describe the setting up of communications between a single phone or an enterprise PBX/IP PBX and the Public Service Telephone Network(PSTN) by an Internet Telephony Service Provider(ITSP). Though a trunk in traditional telephony included only voice, a SIP Trunk may enable multiple voice sessions or concurrency, video sessions, Instant Messaging, presence applications, and other applications.
Wikipedia offers a more detailed description (link)