Voice over IP The ability of an IP network to carry telephone voice signals as IP packets in compliance with International Telecommunications Union Telecommunication Standardization Sector (ITU-T) specification H.323. VoIP enables a router to transmit telephone calls and faxes over the Internet with no loss in functionality, reliability, or voice quality.
From : www.trapezenetworks.com/technology/glossary_6.asp

VoIP stands for Voice Over Internet Protocol. Another term for VoIP is Voice Over IP or Internet telephony. In general, VoIP is converting analog voice data into digital packets and sending those packets over the Internet to a receiver at the other end.
From : www.voipresource.net/VoIP-tutorial.htm