eli5: What’s the difference between lethal, fatal, and deadly?
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In meaning? Nothing. As words? Some are used more commonly in different fields and differ in level of formality.
Lethal – action with intent to kill, object with the ability to kill
Fatal – anything thay caused death is fatal, but the thing itself doesn’t have to be inherently bad (the car crash was fatal, it wasn’t lethal), i feel like this is a past tense, whatever caused the death was fatal, but nothing that’s *just* about to cause death is fatal
Deadly – similar to fatal, any action or object which can cause death
That’s my understanding, english is my second language
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Lethal: high potential to kill. “The gas is lethal when it’s breathed.”
Fatal: has the quality of bringing about death. “His wounds were fatal.”
Deadly: could describe something that will kill or sometimes something specific that certainly killed. “A fall from this height is deadly.” “The pilot’s decision to not check the fuel before departing proved to be deadly.”
These are all considered synonyms.
_Lethal_ and _deadly_ mean tending to cause death when used against or taken by a target. Think of a _lethal weapon_ or a _deadly poison_.
_Fatal_ means has already caused, or is certain to cause, death. Think of _fatal injuries_. An event or occurrence can be fatal; a tool or object cannot.
You could never have a _fatal weapon_, for instance.
My take is that lethal usually refers to a quality of an event or object, while fatal usually refers to an event based on its outcome. Eg a lethal dose, a fatal blow/accident. Lethal referring to an innate characteristic of the item or action, fatal referring to the outcome. A deadly or lethal snake may not have killed anyone yet, but a fatal attack must have.
Lethal-enough to kill most people
A lethal dose of poison could still be survived in rare cases
Fatal-killed someone
The car crash was fatal (the driver was killed)
It’s more definite. The ie a lethal dose could be survived, or it could be fatal.
Deadly-a high potential to kill
The plane crash was deadly (some people were killed, but there were still survivors)
More of a hypothetical, rather than an exact measurement like lethal.
Rule of thumb: Deadly means there was precedence that it killed someone, lethal means that specifically this thing hasn’t, but in principle can kill, fatal means certain death.
You can look at the etymology to get a sense of the meaning: lethal is from Latin meaning “deadly” (so same as deadly, but since it’s a Latin loanword it’s used in a more technical/scientific context and because of science being principled and predictive it refers to being “deadly in principle”), fatal is a derivative (also a loan!) of “fated [to die]”, so certainly deadly and also a more technical term.
EDIT: Here’s an example ->Let’s say you have a high pressure gas tank that is above 250 bar pressure. If this tank catastrophically malfunctions or breaks open, the explosion from the overpressure is lethal (not fatal!). In principle being near such an explosion is pretty much not something you can survive, so in majority cases the wounds you gain would be fatal.
Lethality is in principle deadly, because no one can specifically say that in that specific case that specific person will certainly die, but there is a close to certainty about it (it would require a freak accident to survive).
Fatality is certainly deadly, because a doctor can look at the wounds or make a prognosis, that such wounds/symptoms are too severe to treat, so death is “fated”.
For technical/scientific context we need these distinctions to know whether someone can potentially be saved/helped or not. Someone with a fatal prognosis goes to hospice while someone brought to the ER from a lethal situation is quickly diagnosed and treated if possible, ALTHOUGH we are only this strict on paper, i.e in reports, manuals, warning signs or in journalism.
They’re effectively synonyms, though maybe there are some differences in common usage that can make one more apt or awkward in certain cases than another.
Fatal – mostly used in the sense of “caused fatalities”. A fatal car crash is one in which people actually died. Fatal can also be used euphemistically to describe something that is doomed, or critical. A “fatal flaw” or “fatal error”, is an unrecoverable one.
Lethal – having the potential to cause death, whether it actually has or not. A “lethal weapon” or “lethal injection” is dangerous enough to kill. It can also be used to represent different degrees than fatal. For example, you might say “the newly discovered variant of the virus is more lethal than the original”. In that example, you could interpret the nuance of using lethal vs fatal as suggesting the new virus is more capable of killing, rather than saying it has actually killed fewer people.
It can also be used euphemistically to describe something potent but not necessarily violent – “a lethal stare”, with a similar meaning to “a look that could kill”.
Deadly is, perhaps, somewhere in between and the most flexible. “Deadly poison” is one that has the potential to cause death (like lethal). Whereas a “deadly car crash” is one where people actually died. “Deadly force” could mean either, depending on context.
To my ears, deadly is also the most casual of the three synonyms, and often more literary. “The killer struck a deadly blow”. It is also because it is often is used to mean “very” or “somberly” or “severe”. “I’m deadly serious”. “Seven deadly sins”.
If it was a “deadly day for the navy”, you would interpret that to mean there had been some deaths, but not that the navy itself was destroyed.
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