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

operation code
op code

    (Always "op code" when spoken) 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)