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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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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. May 30, 2013 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. May 30, 2013 at 22:13
  • @BraddSzonye: That's perfect, Bradd. So I guess both the sentences in my example are wrong.
    – Bravo
    May 31, 2013 at 9:33
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    @Shyam I think “lethal cancer” is fine, but “fatal weapon” is probably better written as “deadly” or “lethal.” May 31, 2013 at 9:38
  • Merriam-Webster also has usage information. Broadly similar to the above, but for deadly they emphasise "applies to an established or very likely cause of death", which I think is more accurate.
    – Stuart F
    Feb 27, 2023 at 14:26
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In 1987, My grandfather decided to brag to me a story about his Military days. Halfway through his story, a fellow soldier from the same Battle/War overheard our talking and decided to correct the usage of certain terms being said by my grandfather. This is what the man had said..

Something being LETHAL, means it has the "Potential" to create an automatic kill, but is not automatically able to kill. Meaning, it is able to kill, but is not automatically a kill, as in an Allergy, it is potentially an automatic killer, but can be stopped if given a chance.

Something that is Deadly, is already automatically capable of killing. Deadly, means it cannot be stopped from making its kill (aka an unstoppable Kill). Poisons come in Lethal and Deadly, meaning one has the chance to be stopped from making its kill (has a cure or antidote), while the other will be unstoppable (No cure, no antidote).

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    What about 'fatal'? Feb 27, 2023 at 13:39

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