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This cropped up when I was in a conversation with a friend. I guess fatal must talk of something which has necessarily resulted in death, while lethality is more about potential to cause death. Yet I am not convinced by this explanation, because lethality is associated with specific agents of death, like injection, dosage of medicine, weapons, etc.

Could someone pin the exact difference in usage between these words? Are the following sentences alright, for instance?

  • He has been diagnosed with a lethal type of cancer.
  • He died after getting hit by the fatal weapon.
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up vote 15 down vote accepted

Your understanding is already close to the mark. There's a discussion of the synonyms for fatal in the American Heritage Dictionary:

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.

Thus, fatal and mortal are more often used to describe the immediate circumstances of death, whereas lethal is used more to describe agency. For example, you use a lethal weapon to strike a fatal blow.

Note also that fatal doesn't always refer to death: As a word relating to fate and doom, it's applicable to any catastrophic outcome, not just deadly ones. For example, any plan can have a fatal flaw, but only deadly flaws are lethal.

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This definition makes the potentially useful point that mortal tends to refer to the past, where fatal (which emphasizes inevitability) can reference the past or the future, and deadly/lethal tend to refer to the future. –  FumbleFingers May 30 '13 at 22:09
    
@FumbleFingers Thanks! That's an interesting alternative take on it. It also emphasizes the inevitability of fatal, which is another subtle difference between the words. –  Bradd Szonye May 30 '13 at 22:13
    
@BraddSzonye: That's perfect, Bradd. So I guess both the sentences in my example are wrong. –  Bravo May 31 '13 at 9:33
    
@Shyam I think “lethal cancer” is fine, but “fatal weapon” is probably better written as “deadly” or “lethal.” –  Bradd Szonye May 31 '13 at 9:38
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