The adjective fatal describes something that is capable of causing death. Certain cleaning products, for instance, are labeled, “Could be fatal if swallowed.” Here's a helpful suggestion: don't swallow them.
Can you hear the word fate in fatal? In ancient times, people believed that the future was determined by a force called fate, and in the 14th century, the French used the word fatal to describe anything that fate had decreed, including death. By the next century the word had evolved to mean something that specifically causes death. Today the term is used more loosely. If your friend makes a fatal mistake on the tennis court, it doesn't mean that he'll literally die, just that he'll lose the game.