fa·tal 
(fātl)
(fātl)adj.
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, lethal, mortal
These adjectives apply to what causes or is likely to cause death. Fatal describes conditions, circumstances, or events that have already caused death or are virtually certain to do so in the future: a fatal accident; a fatal illness.
Deadly means capable of killing or of being used to kill: a deadly poison; a deadly weapon.
Lethal has a similar range, often with a suggestion of deliberate or calculated intent: execution by lethal injection; the lethal technology of modern warfare.
Mortal describes a condition or action that produces death, typically in a context of combat: a mortal wound; delivered a mortal blow.
These adjectives apply to what causes or is likely to cause death. Fatal describes conditions, circumstances, or events that have already caused death or are virtually certain to do so in the future: a fatal accident; a fatal illness.
Deadly means capable of killing or of being used to kill: a deadly poison; a deadly weapon.
Lethal has a similar range, often with a suggestion of deliberate or calculated intent: execution by lethal injection; the lethal technology of modern warfare.
Mortal describes a condition or action that produces death, typically in a context of combat: a mortal wound; delivered a mortal blow.