Impaired

Typically a monster's movement follows a predictable pattern. When the monster is affected by certain properties, or it believes the player is in a different square to his actual location, these patterns can be disrupted. Such a monster is said to be impaired. This is a useful shorthand when describing a monster's usual behaviour, by adding the caveat "unless the foo is impaired".

Conflict is not considered an impairment, but a separate category of its own.

Impairing monster properties
The following monster properties can cause untypical monster behaviour.

Confused
A confused monster will move randomly, and may blunder into and attack a player it might not otherwise, unless he is protected by Elbereth, a scroll of scare monster or sanctuary.

Stunned
The NetHack movement subroutines include code to modify monster behaviour when stunned. It is not clear what effect (if any) this produces in the result of these routines.

Scared
Generally, monsters that are scared run away from you. This may cause them to use a ranged attack they would not otherwise have used.

Blind
Monsters with eyes that are blind will move randomly, in a fashion similar to confused monsters.

Player properties that impair monsters
If you are invisible or displaced, monsters that do not yet know your true position will assume you are on a square near to your current location, and behave accordingly.

Invisibility
Monsters will be unable to locate invisible players, unless they can see invisible, they have infravision and the player's current form is infravisible, or they have hit the player since he last moved. They will assume the player is on a square within 1 square of his current location and move accordingly. Generally they will end up next to the player's assumed location, so use of ranged attacks on adjacent invisible players is not common.

Displacement
Monsters will be unable to locate players whose image is displaced, unless they have hit the player since he last moved. They will assume the player is on a square within 2 squares of his current location and move accordingly. This makes it less likely that monsters will score a melee hit, but makes the use of ranged attacks possible.