User Agent Client-
Short for user-agent client. In VoIP, a client application in an SIP system that initiates the SIP request that is sent to the UAS. The combination of the UAC and the UAS is called the SIP user agent. The SIP user agent allows peer-to-peer calls to be made using a client-server protocol.
Source:http://www.webopedia.com/TERM/U/UAC.html