An Acronym for Complex Instruction Set Computing (or chip?)
Describes a CPU that has a large number of machine level instructions hard-coded into it. The most well-known example is the Intel x86 family - each revision has introduced new instructions (for example, MMX extensions) so that the number of instructions in the chip is now well over several hundred.
Compare with RISC (R = reduced)
See 
http://www.arstechnica.com/cpu/4q99/risc-cisc/rvc-1.html for a more in-depth discussion.
