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

orthogonal instruction set

    An instruction set where all (or most)
   instructions have the same format and all registers and
   addressing modes can be used interchangeably - the choices
   of op code, register, and addressing mode are mutually
   independent (loosely speaking, the choices are
   "orthogonal").  This contrasts with some early Intel
   microprocessors where only certain registers could be used
   by certain instructions.

   Examples include the PDP-11, 680x0, ARM, VAX.

   (2002-06-26)