I've read this question, but none of them are what I am looking for. I once read a book that used a word to describe an event that has happened in the past, but the odds were so astronomical that it will likely never happen a second time. The context I read these words in were the creation of the first life on earth (without any theological ideas). Basically, the text said that the scenario for the creation of the first living organism happened on a planet where conditions were just right by complete chance, and that the odds of this were so astronomical that it would likely never happen again anywhere in the universe over any length of time.

Sample sentence: "The creation of the first organisms was incredibly unlikely, the event can be called a _________ event, or an event that will likely never happen again."

I can't pretend to understand the validity of the claims by the text, but the words they used to describe these events were quite clever (and scientific sounding) that it would make a fun hyperbole.

I thought it was something along the lines of singularity event, but that doesn't sound quite right.

  • 11
    Unique would work.
    – Xanne
    Aug 12, 2019 at 18:26
  • 5
    I don’t think it perfectly fits in this context, but anomaly might work in others Aug 12, 2019 at 21:35
  • 7
    A black swan? Aug 12, 2019 at 22:13
  • 3
    The word "fluke" refers to an event which occured primarily due to chance and is unlikely to occur, although perhaps not to the extent of not occurring ever again.
    – StockB
    Aug 13, 2019 at 13:54
  • 2
    black swan sounds right to me, since it's also the title of a book about the subject: en.wikipedia.org/wiki/…
    – Eevee
    Aug 15, 2019 at 2:57

13 Answers 13


The Century Dictionary and Cyclopedia defines Singular as meaning:

  1. Out of the usual course; unusual, uncommon; somewhat strange; a little extraordinary: As a singular phenomenon

More literally it derives from single, meaning one, so a singular phenomena is logistically something that is assessed as happening just once. It is of course possible to use the term hyperbolicly or mistakenly, which can diminish the effect, but I doubt you will find any word that lacks that particular problem. The nature of hyperbole is that people tend to exaggerate, and sometimes intentionally, so they will choose whichever word most effectively achieves the exaggeration they want to convey.

Since the dictionary entry was written, singular event has grown more popular than phenomenon as shown by Google Ngrams, and Collins shows that singular is one of the top 10,000 words used in the language, so I figure that this demonstrates that it is more than common and easily enough understood.

This chart demonstrates the popularity of singular event, singular events, singular phonomenon and singular phenomena. While singular phenomenon was the most popular, the other three had relatively even use in the 1820s. The usage of all four declined usage until the 1940s, but afterwards both the event and events began to rise in popularity again while phenomena remained stable, making Singular event the more popular forms by the end of the graph at 2008.

An example of relevant usage used in a context regarding the unlikely creation of life can be found on page 3 of Free Radicals: Biology and Detection by Spinn Trapping et al (1999):

This biblical account of creation [referring to an excluded quotation of Genesis 1:1–7], a theme common to the cultural life of many societies over the past several millennia (Westerman, 1974, foretells the birth of life on our planet. From the available evidence we have today, this epic began about 15 billion years ago, when, we are told, a massive explosion of incomprehensible power, referred to as "The Big Bang," created everything that was, is, and will be (Hawking, 1088; Peebles et al., 1994). From this singular event, the evolution of life commenced with the formation of hydrogen and, to a lesser extent, helium.

Regarding ❌singularity event, the reason that does not work is because the -ity suffix functions to change the adjective into a noun, as noted by the entry for -ity:

A common termination of nouns of Latin origin or formed after Latin analogy, from adjectives, properly from adjectives of Latin origin or type, as in *activity, civility, suavity etc., but also in some words from adjectives not from Latin origin or type, as in jollity. The suffix is properly -ty, the preceding vowel belonging originally to the adjective. See -ty2.

Sometimes adjoined nouns do modify nouns, but much more rarely than adjectives and in a very different manner.

  • 2
    I've googled for weeks looking for something that sounds and feels right, and in less than 24 hours you've linked me to an article that uses the exact same context as my sample sentence. Thank you for the quick lesson on the suffixes; I don't remember going that in-depth with them in school.
    – Thaddeus
    Aug 14, 2019 at 12:40

The creation of the first organisms were incredibly unlikely, the event can be called a one-off event, or an event that will likely never happen again.

Merriam Webster defines it as

limited to a single time, occasion, or instance

A witty alternative would be once in a lifetime but I find it rather confusing in this context. It might work for shorter timescales, though.

  • That is probably the word the text should have used, as that is the sole purpose of the word, but the reason I remembered reading this text was because I asked myself "Why didn't the author use one-off?" Although for anyone reading the question, one-off is definitely the word that should be used.
    – Thaddeus
    Aug 12, 2019 at 18:00
  • The word I am looking for has a more scientific connotation behind it.
    – Thaddeus
    Aug 12, 2019 at 18:02
  • "One-off" sounds best to me, though rather informal, since "singular" implies something like "remarkable". "Unique" is another alternative maybe but doesn't necessarily mean it happens once, only that the event is unique. Aug 14, 2019 at 23:56


Unique means ‘singular, or only happening once.’

It is from the word one - etymology - ‘early 17th century: from French, from Latin unicus, from unus ‘one’.’

A unique event. One that happens only once.

Unique has also been broadened to mean ‘unusual or rare’ but then, things that only happen once, generally are that.


  • 1
    I would say that this more applies to the event rather than the frequency. E.G. The Cheese-rolling race in the UK could be described as a "unique" event, but it happens most years (though many people wonder why...) Aug 14, 2019 at 23:58
  • That’s exactly what I mean when I say ‘Unique has also been broadened to mean ‘unusual or rare’ but then, things that only happen once, generally are that.’
    – Jelila
    Aug 18, 2019 at 2:04

Two other suggestions. First, there is the latin phrase sui generis meaning the same as "singular" or "one-off" (see other answers).

For the most extreme degree of unlikelyhood short of actual impossibility, there is the scientific phrase "a thermodynamic improbability". This refers to an event which is not actually forbidden by the laws of the universe, but which is so exceptionally unlikely that it will "never" happen -- unless the context is an infinity of time and space, in which case it is certain to happen, because infinity divided by any finite number is still infinite. An example would be one's head spontaneously falling off, simply because all the atoms comprising one's head happened to vibrate in the same direction at the same time.


Perhaps isolated, as something that shouldn't happen again, but does not guarantee it will not reoccur:

"The high school students assured their principal that the cafeteria food fight was an isolated incident, promising that it would never, ever happen again."


Used in medical terminology as well:

"Isolated occurrence means a single event which a physician concludes with reasonable medical certainty will not recur in the future"


  • To me that just means "not connected to other occurrences". Food fights in cafeterias occur all the time, but are not necessarily related in any way. Not quite the same as what the OP was asking. Aug 13, 2019 at 20:48
  • @DarrelHoffman "Isolated" is more like "far from other occurences". "Not connected to other occurrences" would be "independent". Aug 14, 2019 at 11:21

"The creation of the first organisms was incredibly unlikely, the event can be called an unwonted event, or an event that will likely never happen again."

Unwonted: (M-W Dictionary)

1: being out of the ordinary :RARE, UNUSUAL

2: not accustomed by experience


My first thought was a "freak event":

A freak event or action is one that is a very unusual or extreme example of its type.

(source: Collins Dictionary)

I don't believe it has the same negative connotation when applied to an event as it does when applied to a person.


I'd suggest fluke: An event that happened by a stroke of luck, or accident, rather than planning. It has a positive connotation which is proper for the context of emerging life.

The word is informal; but this lightness may fit for an event which was not planned or predictable, even in an otherwise serious text.


"Singleton" was the first word that popped into my head. Although I've never heard it used outside of computer programming.

  • Same root as "singular" above. "Singleton" is programming jargon (and possibly maths), not in general use.
    – nigel222
    Aug 13, 2019 at 9:48
  • 3
    In math, singleton is a set with one element. In computer science is a class with only one instance. In other words, computer scientists used the definition from math. Aug 13, 2019 at 22:01
  • 3
    A singleton is also a term used in cards, to mean only one card of a particular suit is held by a player. Aug 14, 2019 at 14:05
  • If described as a milestone“ singularity“ (like AI reaching self awareness) is used.
    – eckes
    Aug 14, 2019 at 20:10
  • 1
    Singleton is not programming jargon, it's an English word that's been in use for centuries, which was adopted as programming jargon only recently.
    – barbecue
    Aug 15, 2019 at 15:46

A purple unicorn.

A unicorn being, mythically, something so extraordinary that one would never get to see it, coupled with the added distinction of ‘purple’ - making it even more rare and thus impossible to ever be seen.

So I think if you describe your event as a ‘purple unicorn’, people will get that it is too extraordinarily rare for it to ever actually occur again.


  • Please don't copy answers. If an answer is applicable to two questions, vote to close one as a duplicate of the other. Duplicate answers raise automatic flags.
    – Andrew Leach
    Aug 14, 2019 at 20:43
  • They are different questions. My answer happens to apply to both of them. What should I do?
    – Jelila
    Aug 18, 2019 at 2:05

It’s not a single word, but a often used phrase

once in a lifetime

it won’t match events which occur less often, I admit.


I know other people have already suggested 'one-off,' but the phrase that immediately jumps to my mind is "A one-time thing." It was a one-time thing - it happened once, and it's never going to happen again.


From John Lawler's upvoted comment, one option is black swan.

black swan    

an event that is extremely rare and unexpected but has very significant consequences

The problem with black swan events is that very often they are never repeated.

Word Story:

When Dutch explorer Willem de Vlamingh first saw black swans in Western Australia in the 17th century, Europeans believed that all swans were white, so a black swan seemed an impossibility. The term came to be used for an event that happens even though it seems impossible.

Source: Macmillan Dictionary, https://www.macmillandictionary.com/us/dictionary/american/black-swan

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