I'm looking for a single word that means "almost certainly" but leaves room for 'technical' or 'philosophical' doubt. That is, something like 99.99999...% certain. Or "close enough to certain that we don't need to get hung up on it".

  • probably doesn't work because it just means over 50% – way too low.
  • indubitably doesn't work because it literally means "undoubtable", i.e. its definition explicitly excludes the possibility of any doubt.
  • definitely, certainly, doubtless – strictly speaking, these do not allow for any doubt either (even though they are often used in casual speech for things that aren't truly certain). I want a word that, when used precisely and literally according to its dictionary definition, perhaps in a philosophical/technical/engineering context, actually means "certain, except for the usual 0.00000...1% doubt".

The word should acknowledge a shade of doubt without getting stuck on it, if that makes sense. I don't know if such a word exists, but it seems odd to me if it doesn't, as it seems like it would be very useful in epistemological and engineering discussions.

I guess "beyond reasonable doubt" is a common phrase that means roughly what I want, but I'm wondering if there is a word that (a) is a single word or simple compound, and (b) is used in more of a philosophical/technical/engineering/critical thinking context than a legal one.

  • 1
    Try doubtless.
    – Robusto
    Commented Oct 29, 2021 at 0:38
  • 1
    From the description of the tag "single-word-requests": To ensure that your question is not closed as off-topic, please be specific about the intended use of the word. INCLUDE A SAMPLE SENTENCE demonstrating how the word would be used. Commented Oct 29, 2021 at 2:18
  • 1
    Also, thesaurus.com/browse/probably Commented Oct 29, 2021 at 2:21
  • Indubitably there is.
    – Jim
    Commented Oct 29, 2021 at 4:23
  • Is this general usage, or the technical usage of almost certainly in probability theory?
    – The Photon
    Commented Oct 29, 2021 at 4:51

5 Answers 5


If you have no specific potential doubt in mind then it's 'as certain as blow it'. But that may be too casual/informal for your purpose.

If something is 99.999.... certain, then it is as 'near certain as makes no matter'. but if the difference makes no matter, then you have no reason to doubt it at all. And in that case, you may as well say it is certain.

The point 0.0000000...... chance of being wrong is so small that you never get to a 1 at the end. If if such an improbability were to occur, nobody could possibly calculate its probability. It would be like the probability of all the atoms in the chair you are sitting on moving upwards at the same instant. We are talking about the infinite improbability drive in 'A Hitch-hiker's Guide to the Galaxy. After all. 0.00000.... is infinite improbability.

Short of that, 'nearly', 'almost' or 'virtually' certain are all fine. In real life, if the virtually impossible does happen, it will not help you to plead that in fact you were right on the grounds that it it was, in fact, virtually impossible, unless there is a precisely calculable probability. In which case, I guarantee it will be at least 1% and probably more.


Since you're mentioning engineering, "with infinitesimal likelihood to fail", "tangentially close to target", and "asymptotically near certainty" could all be candidates for what you mean.

Note that these expressions really only fit into a scientific context, and I wouldn't expect to hear them in layman usage.

I do wonder if there's an antonym for "infinitesimal" which could be used here.

  • 2
    But OP also specifies a single word answer. And ELU deals with standard usages, mazunki, not technical language (especially not constructs that get zero hits in Google searches). Commented Apr 18, 2022 at 16:10

1 as submodifier Nearly; almost.

Source: Lexico

As in the statement “I am virtually certain.”

  • This would be improved with a dictionary definition Commented Jun 17, 2022 at 14:59

From Dictionary.com:

Presumptive [adjective]

  1. affording ground for belief or presumption: presumptive evidence.

  2. based on likelihood or presumption: a presumptive title; the presumptive nominee.

  3. regarded as such by presumption; based on inference; assumed: a presumptive case of pneumonia.

  • 1
    Nice choice for not expressing full certainty. But since "almost certainly" is an adverbial phrase, maybe "presumably" would be a better fit? Commented Oct 29, 2021 at 2:20
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    No. 'Presumptive' does not entail the '99.99999...% certain' requirement. Commented Oct 29, 2021 at 14:39

My word that suggests a virtual likelihood is the word, "Invariably".

In many dictionaries today, this word has come to mean, "always", but somehow the definition has "invariably" come to mean always. Yet, the synonyms, suggest otherwise! Tough call, but when using this word with the older crowd, or those who have spent a lot of time reading and composing over the past 50, 60 or more years, have acknowledged this trend, hence the word meaning 100 percent.

When using this word in a phrase, it suggests a virtual 100 percent (99.99999999999999999) probability of something happening, however there is just enough wiggle-room to suggest that it can on very very rare occasion, not be a true statement. IT is the aformentioned that led me to make a comment regarding the OP's question!

These days, a statement will have the words, "almost invariably", once again, making the word itself and the argument for its meaning pretty strong for meaning, "always"!

  • No, invariable means what it says - 100% of the time. Also, virtual means unreal, so when you say virtual 100% that means 0%, not >99%.
    – Chenmunka
    Commented Nov 8, 2023 at 9:14

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