An impact weapon is one of the six hitscan weapons that will operate a "gunfire activated" linedef, such as a wall that opens upon being shot.
Specifically, the impact weapons found in Doom are:
Impact Weapons In Other Games[]
Heretic[]
The following impact weapons appear in Heretic:
Hexen[]
The following impact weapons appear in Hexen:
Strife[]
The following impact weapons appear in Strife: