Cheating hacks are any modification of the user's system, of the game executable directly, or of the executable through another application, that is thus not normally available in Doom and provides an advantage to one player over others in multiplayer games or speedrunning events, with regard to:

• humanly-executable player movement,
• visual abilities,
• audio abilities,
• player aim abilities,
• mechanisms to locate the opponent,
• features that slow the game to the player's advantage, or
• the possibility to redo portions of a challenge.

These changes are put into effect directly or indirectly against another player who either

• does not agree to, or
• is unaware that the changes have been implemented.

Modifications to sound and graphics to help a hearing or visually impaired player compensate for his or her disability are usually not considered cheats.

A number of methods exist that can be used for cheating in Doom:

• Aimbots
• Map editing
• SR-50 automation
• No red
• Full brightness
• Sprite editing
• Sound editing
• Red dot
• Slow motion
• Recording from a saved game
• Built demos

Source

This writeup includes text from An Explanation of cheating in Doom2 deathmatch, used with permission.