Extended Instruction Set
From Computer History Wiki
Extended Instruction Set was the name given to a number of optional additions, to different models from different manufacturers, to their CPUs; those added instructions to the basic set provided by the CPU of the 'base' model of any line.
(This concept obviously only made sense when CPUs were physically large constructs, made up of many discrete components, such as transistors or SSI chips. Only the very earliest microprocessors had such options, often floating point arithmetic units.)
