ATSC (short for Advanced Television Systems Committee) is a large consortium (constituted by ~140 members from the broadcast, broadcast equipment, motion picture, consumer electronics, computer, cable, satellite, and semiconductor industries) that develops a set of standards related to Digital TV.
In casual conversation or less precise descriptions, the terms "ATSC" or "ATSC Standard" are used generically to refer to "ATSC Digital Television (DTV) Standard (A/53)", which is the ATSC developed standard for terrestrial based over-the-air (OTA) broadcasts, and which has been adopted by the U.S.A, Canada, Mexico, South Korea and perhaps some other countries.
Similar to other DTV systems, A/53 calls for the transmission of a MPEG-2 Transport Stream, but uses the 8VSB modulation scheme, instead of COFDM like the competing DVB-T counterpart. During the DTV trial stages, and as well as for near close to a decade following the 1996 U.S selection of the ATSC DTV Standard for use in terrestrial transmissions, a kind of holy war has been waged in some circles as to whether which of DVB-T or ATSC is the technically more advanced system. For one such analysis of these two systems, see here. In 2001, a report compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8VSB. Much of this debate has subsided over the course of time.