1. Causing or capable of causing death.
2. Causing ruin or destruction; disastrous: "Such doctrines, if true, would be absolutely fatal to my theory" (Charles Darwin).
3. Of decisive importance; fateful: came through at the fatal moment.
4. Concerning or determining one's fate: the fatal thread of life.
5. Obsolete. Having been destined; fated.

[Middle English, fateful, from Old French, from Latin fātālis, from fātum, prophecy, doom. See fate.]

SYNONYMS   fatal, deadly, mortal, lethal. These adjectives apply to what causes or is likely to cause death. Fatal describes conditions, circumstances, or events that have caused or are destined to cause death or dire consequences: a fatal illness. Deadly means capable of killing: a deadly poison. Mortal describes a condition or action that produces death: a mortal wound. Lethal refers to a sure agent of death that may have been created solely for the purpose of killing: execution by lethal injection.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.