A stage hazard, more commonly referred to as a hazard, refers to a particular stage element in Super Smash Flash 2 that can directly interfere with the characters as they are fighting in the middle of a match. Most stage hazards can directly damage and launch characters on contact, like the lava in Phase 8 or the red beams from Lunar Core, while other may be sudden changes or transitions on multiple stages' layouts, like Galaxy Tours or Castle Siege, and sometimes some may be more of a nuisance rather than an actual threat, like the drawers from Desk or the bumpers in Casino Night Zone. Some stage hazards normally come at random intervals, making them quite unpredictable at times.
All stage hazards, and certain stages' changes, can be disabled with the help of the hazard switch found directly on the stage selection screen, thereby allowing for more open stage options in tournaments where the a stage hazard can no longer play a role in disallowing the stage itself.
None of the regular stages in the original Super Smash Flash present hazards at all. One can argue, though, enemies found in Adventure mode's levels can somehow be technically considered stage hazards.