WordWeb Online

1. Bringing death
   - killing
    
2. Having momentous consequences; of decisive importance
   "the fatal day of the election finally arrived";
   - fateful
    
3. (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";
   - black, calamitous, disastrous, fateful
    
4. Controlled or decreed by fate; predetermined
   "a fatal series of events";
   - fateful

See also: deadly, deathly, decisive, inevitable, lethal, mortal, terminal, unfortunate

Antonym: nonfatal

Encyclopedia: Fatal