Any substance that, when present inside a creature in sufficient quantity, harms that creature through chemical reaction or similar interaction on a molecular scale.
Poisons come in many types and flavors depending on the game system. They generally:
-Cause either direct damage or disability or both
-Can be potentially resisted or overcome by both mundane or magical means typically (obvious exceptions still occur)
-May be administered in a small variety of ways such as ingested, inhaled, or introduced via damage in combat. Not all poisons have multiple means of administration.