The Free On-line Dictionary of Computing (30 December 2018):

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)