This term can have several meanings, depending on the context:
(1) Microcomputer released in 1984 by IBM based on the 80286 microprocessor. Here AT meant Advanced Technology.
(2) Motherboard form factor introduced by this computer that became the "de facto" standard for the motherboard industry, largely adopted by PC-clone manufacturers. The original AT measured measured 13.5" x 12" (34,3 cm x 30,5 cm) and several smaller versions were created, generically called Mini-AT or Baby-AT. Usually Mini-AT motherboards measured 13" x 8.7" (33 cm x 22 cm) but since this format wasn't standard many manufacturers built their motherboards with different sizes. This form factor was used for several years until the release of the ATX form factor, that replaced the old AT standard.
(3) Power supply standard used by AT motherboards.
(4) Case standard used by AT motherboards.
(5) Keyboard standard used by AT motherboards.