(1) Short for advanced technology, the AT is an IBM PC model introduced in 1984. It includes an Intel 80286 microprocessor, a 1.2MB floppy drive, and an 84-key AT keyboard.
Today, the term is used more generally to refer to any PC with an 80286 processor.
Also see From AT to BTX: Motherboard Form Factors in Webopedia’s “Did You Know…?” section.
(2) See @ sign.
(3) Short for assistive technology.