From Sega Retro
When a ROM (or part of a ROM) is converted from raw machine code into readable assembly code, it is said to be a disassembly. For example, 4E75 in a ROM would disassemble to rts in Motorola 68000 assembly, which means "return from subroutine".
Programs which can disassemble Motorola 68000 machine code include 68kd and IDA Pro. 68kd doesn't produce assembly code which can be recompiled by SNASM68K. IDA Pro does produce recompilable code, but it isn't freeware.