VAX - A 32-bit computing architecture that supports a very orthogonal machine language and virtual addressing (i.e. demand paged virtual memory). It is the quintessential CISC processing architecture, including machine instructions for queue insertion/deletion and polynomial evaluation.
VAX computer systems (informal plural is VAXen) could run under several operating systems, usually BSD UNIX or VAX/VMS by Digital Equipment Corporation (later purchased by Compaq Computer Corporation). The VAX architecture and VMS operating system were "engineered concurrently" to take advantage of each other, including sophisticated clustering, initially over special CI (computer interconnect) busses but later over Ethernet as well.
VAX was originally an acronym for "Virtual Address eXtension" because the VAX was seen as a 32-bit extension of the older 16-bit PDP-11 -- early versions of the VAX processor implemented a "compatibility mode" that emulated many of the PDP-11 instructions. Later versions offloaded the compatibility mode and some of the less used CISC instructions to microcode or emulation in the operating system software.
The first VAX model sold was the 11-780 which became available circa 1978. Many different models with different prices, performance levels, and capacities were made. VAX minicomputers were very popular in the early 1980s. In 2001 there are still VAXs doing useful work, and Compaq was even selling a tiny number of new ones until recently (perhaps still are).
For a while the VAX 11-780 was used as a baseline in CPU benchmarks because its speed was about one MIP. Entertainingly enough, the actual number of instructions executed in 1 second was about 500,000. One VAX MIP would be the speed of a VAX 11-780; a computer performing at 27 VAX MIPS would run the same program roughly 27 times faster than the VAX 11-780. Within the Digital community the term VUP (VAX Unit of Processing) was the more common term, because MIPS do not compare well across different architectures.
The VAX went through many different implementations. The original VAX was implemented in ECL, and filled more than one rack for a single CPU. The final versions were implemented in CMOS.