ATSC stands for Advanced Television Standards Committee. This organization is charged with organizing specifications and requirements for High Definition and Digital Television performance and broadcast standards. Its recommendations are usually adopted by in the U.S. by the FCC (Federal Communications Commission)
As a result of the ATSC's partnership with the FCC, all HDTVs manufactured for the U.S. market that include built-in tuners, have to adhere to the technical requirements of the ATSC, as approved by the FCC. As a result, HDTV tuners are referred to as ATSC tuners.
For more details on the ATSC, check out their website: ATSC.org.