Fatal means causing or capable of causing death. A fatal accident is one where someone dies. A fatal disease is one that leads to death. If a mistake is fatal, it results in death or complete disaster.

Fatal errors in judgment can doom entire plans. A general who makes a fatal mistake in battle might lose not just the fight but the entire war. A mountain climber's fatal decision to ignore storm warnings could end in tragedy. The word captures that sense of finality: once something fatal happens, there's no going back.

You'll also hear fatal used more loosely to mean critically damaging, even when no one dies. A politician's lie might be described as a fatal blow to her career, meaning it destroyed her reputation completely. A football team that fumbles in the final seconds might call it a fatal error, meaning it cost them the game. In these cases, fatal emphasizes how serious and hard to undo the consequences are.