An Acronym for Extended Technology.
The full name is really PC/XT. This is an IBM PC flavour with a (slow) hard disk, and was released in March 1983. It had an Intel 8088 CPU. The XT/370, released in October 1983, added IBM 370 mainframe emulation, and the XT 286 followed in September 1986 with an Intel 80286 CPU.