Source : WordNet®

operation code
     n : the portion of a set of operation descriptions that
         specifies the operation to be performed; the set of
         operations in a computer [syn: {order code}]

Source : Free On-Line Dictionary of Computing

operation code
     
         (Or "op code") The part or parts of a {machine
        language} instruction which determines what kind of action the
        computer should take, e.g. add, jump, load, store.  In any
        particular {instruction set} certain fixed bit positions
        within the instruction word contain the op code, others give
        {parameters} such as the addresses or {registers} involved.
        For example, in a 32-bit instruction the most significant
        eight bits might be the op code giving 256 possible
        operations.
     
        For some instruction sets, certain values in the fixed bit
        positions may select a group of operations and the exact
        operation may depend on other bits within instruction word or
        subsequent words.
     
        When programming in {assembly language}, the op code is
        represented by a readable name called an {instruction
        mnemonic}.
     
        (1997-02-14)