Generally, a number that corresponds to an operation. In machine language, the opcode is the part of the instruction that tells the microprocessor which operation to perform. Usually this information is stored in one or two bitfields.
So, if I have in PowerPC PPCAsm
addi r3, r4, 5
which assembles to binary
001110 00011 00100 0000000000000101
opcode operands immediate data
001110, decimal 14, is the opcode corresponding to the addi mnemonic
and the operation of addition
Bytecode languages and any way of representing algorithms also use opcodes.