AC-3

AC stands for audio compression, but may be referred to as Dolby Digital (Dolby AC-3). 

The digital sound system specified for H/DTV broadcasting. AC-3 delivers 5.1 (front left, front right, centre, surround left and surround right speakers, plus an LFE (low frequency effect) subwoofer) channels of CD-quality digital audio. AC-3 is developed by Dolby Labs.