Source : Free On-Line Dictionary of Computing

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)