An environmental hazard in gaming is when something that is not an enemy AI can inflict damage upon the player.
Examples of environmental hazards
Backdrafts pose a serious hazard in harder difficulties of Resident Evil Outbreak's "Hellfire" scenario. Opening a door with large amounts of smoke billowing out from under it will cause such an event. In HARD and VERY HARD modes, certain rooms (Room 201 in HARD, and the Store Room and Room 301 on VERY HARD, respectively) cannot be accessed as a result.