An opcode (operation code) is the first part of an instruction that is read by the decoder to select the device (circuit) that implements the operations.
It's a unique number that identifies an operation.
Each opcode is a member of the instruction set.
A opcode is a digit. Assembly language gives name to opcode that are called mnemonic.
In a assembly format, simple opcode example
opcode 1100, 1100
# if opcode corresponds to the binary 00, it's the same as
0011001100