From NES Hacker Wiki
In the example below, STA is the opcode, $10 is the operand, and the two combined is the instruction.
0001:85 10 STA $10 ; Stores the accumulator into memory location $10.
This example with RTS only has an opcode, but no operand. Even still, it is an instruction.
0001:4D RTS ; Return to calling subroutine.