It is the common name for a six-channel surround sound audio system. 5.1 is the most commonly used layout in home cinema. It uses five full-bandwidth channels and one low-frequency effects channel (the "point one"). Dolby Digital, DTS, are all common 5.1 systems. 5.1 is also the standard surround sound audio component of digital broadcast and music. All 5.1 systems use the same speaker channels and configuration, having a front left and right, a center channel, two surround channels and a subwoofer. With miniDSP's products you can perfect this system with room acoustic measurements (UMIK-1), or optimizing your bass management (2x4 HD) and even perfect your sound with our room correction product line: Dirac Series (DDRC-88A), nanoAVR HD/HDA/DL products, and the 10x10 HD.
(Source: wikipedia.org)