(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.