No operation. An assembly language instruction in several architectures, used in timing loops, to "comment out" binary code, or to leave space for future code expansion.
The official NOP ($EA) in the 6502
architecture is one byte and two cycles, but there are plenty of other NOPs in the original 6502 opcode matrix
. The one that produces the fastest path through the binary code (one cycle per byte) is STA
See also: 6502 instructions | 6502 addressing modes