A fatal accident or illness causes someone's death. A hospital spokesman said she had suffered a fatal heart attack. + fatally fa·tal·ly The dead soldier is reported to have been fatally wounded in the chest
having momentous consequences; of decisive importance; "that fateful meeting of the U N when it declared war on North Korea"- Saturday Rev; "the fatal day of the election finally arrived"
(of events) having extremely unfortunate or dire consequences; bringing ruin; "the stock market crashed on Black Friday"; "a calamitous defeat"; "the battle was a disastrous end to a disastrous campaign"; "such doctrines, if true, would be absolutely fatal to my theory"- Charles Darwin; "it is fatal to enter any war without the will to win it"- Douglas MacArthur; "a fateful error"
A fatal action has very undesirable effects. It would clearly be fatal for Europe to quarrel seriously with America He made the fatal mistake of compromising early It would deal a fatal blow to his fading chances of success. + fatally fa·tal·ly Failure now could fatally damage his chances in the future