1.Definition: (of events) having extremely unfortunate or dire consequences; bringing ruin
"Such doctrines, if true, would be absolutely fatal to my theory", "It is fatal to enter any war without the will to win it"
2.Definition: bringing death
3.Definition: controlled or decreed by fate; predetermined
"A fatal series of events"
4.Definition: having momentous consequences; of decisive importance
"The fatal day of the election finally arrived"