Source : Free On-Line Dictionary of Computing

instruction set architecture
     
         (ISA) The parts of a {processor}'s design that
        need to be understood in order to write {assembly language},
        such as the {machine language} instructions and {registers}.
        Parts of the architecture that are left to the implementation,
        such as number of {superscalar} {functional units}, {cache}
        size and {cycle} speed, are not part of the ISA.
     
        The definition of {SPARC}, for example, carefully
        distinguishes between an implementation and a specification.
     
        (1999-01-16)