An aid to
reverse engineering.
In terms of software, a disassembly is an assembly language listing created by disassembler program and/or an eager hacker, engineer or hobbyist.
Here is an example of a disassembly from the disassembler included in the MSDOS DEBUG command:
0000:0306 0100 ADD [BX+SI],AX
0000:0308 0000 ADD [BX+SI],AL
0000:030A 52 PUSH DX
0000:030B 27 DAA
0000:030C 93 XCHG BX,AX
0000:030D 0000 ADD [BX+SI],AL
0000:030F F0 LOCK
0000:0310 0000 ADD [BX+SI],AL
0000:0312 2335 AND SI,[DI]
0000:0314 0000 ADD [BX+SI],AL
0000:0316 EE OUT DX,AL
0000:0317 0300 ADD AX,[BX+SI]
0000:0319 002E0300 ADD [0003],CH
0000:031D 00FF ADD BH,BH
0000:031F FF00 INC WORD PTR [BX+SI]
0000:0321 00FF ADD BH,BH
0000:0323 FF00 INC WORD PTR [BX+SI]
0000:0325 1101 ADC [BX+DI],AX