Acronym for Inter Asterisk eXchange protocol. Two versions exist; version 2 is now the default protocol in most modern Asterisk distributions.
IAX is a UDP based protocol, and does not use random port numbers for the voice stream making it nice behind NAT. IAX also supports trunking and transfer of VoIP calls to remote exchanges directly.
See Also: http://www.voip-info.org/wiki-IAX