I have recently found this video online from Merriam Webster saying the plural form of octopus is in fact octopuses. The video explains how octopus comes from the Greek language and thus it would be incorrect to say octopi since that is a latin plural form. The Greek plural of octopus would be octopodes, but octopus actually turns into an English word which makes the plural octopuses.

With that being said, why do dictionaries have octopi listed as the plural form if that is incorrect? Is the plural so widely misused that they just threw in octopi in there?

And then there is the Latin word radius. No one ever uses the word radiuses however it is listed in the dictionary. In fact, even my google chrome and firefox spell checker tells me radiuses is incorrect. Is this word never meant to be used? Despite radius also being an English word, people will always say radii for the plural version.

  • grammarist.com/usage/octopi-octopuses
    – Jim
    Commented Feb 13, 2013 at 5:34
  • 2
    Related: What is the correct plural of octopus?
    – aedia λ
    Commented Feb 13, 2013 at 5:59
  • 1
    It's radii of circles, but radiuses when talking of arm-bones.
    – Andrew Leach
    Commented Feb 13, 2013 at 7:17
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    If not for anything, this question shouldn't be a duplicate. I have indicated in the post that I already understand the correct plural of octopus radius. My question pertains more to the usage of the two words.
    – krikara
    Commented Feb 13, 2013 at 20:24
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    I agree. This is wider, with octopi being an example, to a more general question of what is or isn't considered a "real" word.
    – Jon Hanna
    Commented Feb 14, 2013 at 1:28

4 Answers 4


Octopi is an example of a hypercorrection.

Some people know that English often uses plurals according to the rules of the language a word came from. (It's not strange that this happens, since such words after all aren't English in origin, so the difference is between borrowing a word and then pluralising it, or borrowing the plural word.)

These people also know that with such words, use of the English plural form is sometimes considered incorrect. They further know that, justly or unjustly, where there is a choice, the use of the non-English form is sometimes considered cleverer, because it requires further knowledge.

Virii is another common example, which may have started as a joke. Indeed, several such cases did; penii being a more more obvious example of it being a joke rather than an earnest mistake.

Opus is an interesting case, because opuses and opera are both found, following English and Latin rules respectively, but also opi, following Latin rules incorrectly; so we have both an English, a Latin, and a hypercorrect Latin form.

A hypercorrection, or any other mistake, can eventually become part of a language if followed by enough people. Since a language exists in a medium of loose consensus, then even if we don't like this, it'll be the case. An example that's dying out again (and isn't about plurals) is that for a long time foetus was the more common spelling than fetus, though the latter became the more common spelling in America some time ago, and elsewhere quite recently. Yet foetus is a hypercorrection of what in Latin was originally fetus. Indeed, I'll still spell it foetus even though I know this because it's the spelling I'm used to.

Now, when we come to dictionaries, it depends on the purpose of the dictionary in question.

A dictionary like the OED would be remiss in not mentioning octopi, because octopi is used considerably, and it's the job of the OED to record this.

With other dictionaries, there's a good argument to be made for not including it. Arguments for including it though would be:

  1. Someone who came across octopi and looked it up, should be given an answer (perhaps with a note saying "incorrect" or "proscribed", perhaps not).

  2. The usage panel may have decided that it's so heavily used as to count as a plural that came from an error, rather than an error.

In fact, even my google chrome and firefox spell checker tells me radiuses is incorrect.

This is a case that certainly shouldn't allow octopi; the only reason for a spell-check dictionary is to spell something correctly. Even if we were to edge toward the idea that octopi had been fully absorbed into the language as a new plural, while octopuses remains common and easy to guess at if you're confused by "why did it say that wasn't the word I wanted?" then it serves no real purpose at all.

If anything, spell-check dictionaries are too forgiving for their purposes; if one mistypes and accidentally produces an obscure word it's better to have it flagged even if there's no doubt it is indeed a word; you can add it yourself if you've cause to use it often.

  • Excellent answer. Spell checker finds both radiuses and octopi to be incorrect. Funny how that works out.
    – krikara
    Commented Feb 13, 2013 at 20:16
  • A related question, is whether spell-checks should allow adduct, as both LibreOffice and Firefox's do. Now, AFAIK, the only way to spell adduct is as I have it here, but if it appears in a piece of writing is it more likely to be intended a typo for abduct or a thinko for adduce? I'd say it would be better to let those who are likely to write about adduction add the word (or perhaps select a topic they write about that imports a relevant extra dictionary), and then then people who intended abduct or adduce can have their error flagged.
    – Jon Hanna
    Commented Feb 13, 2013 at 20:27

The OED gives the plurals of octopus as octopuses, octopi, octopodes in that order, describing octopi as ‘rare’. It also describes radiuses as ‘rare’, compared with radii. The choice, however, is one of style. My own preference is to form an English plural with ‘-(e)s’ wherever possible, because I think anything else sounds stilted. So, for me, octopuses, certainly, but there are not many occasions on which I would need to use it. The occasions on which I might require the plural of radius are rarer still, but I think I’d be tempted to use radiuses, just to annoy those who think they know a bit of Latin.

  • In the previous Iraq adventure there were lots of journalist showing off by using "courts martial" as the plural.
    – mgb
    Commented Feb 13, 2013 at 12:57
  • @mgb are you sure "court martials" has over-taken "courts martial" as the plural?
    – Jon Hanna
    Commented Feb 14, 2013 at 1:39
  • @JonHanna - no but it should.
    – mgb
    Commented Feb 14, 2013 at 6:12
  • @mgb. Such evidence as there is favours courts martial, for which there are 22 records in the COCA, 3 in the BNC and 23 in the OED. The figures for court martials are 6, 0 and 3. This Ngram confirms these findings: tinyurl.com/d4qq6tj Commented Feb 14, 2013 at 7:48
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    @mgb when pluralising any noun with a post-positive adjective, it's still generally more common to pluralise the noun rather than the adjective, heirs apparent, Pounds sterling, best rooms available rather than heir apparents, Pound sterlings, best room availables.
    – Jon Hanna
    Commented Feb 14, 2013 at 9:17

Wikipedia acknowledges the issue you being up, and in term mentions that the Oxford English Dictionary does as well.

The Oxford English Dictionary[9] lists octopuses, octopi and octopodes (in that order); it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun, which it is not. Rather, it is (Latinized) Ancient Greek, from oktṓpous (ὀκτώπους), gender masculine, whose plural is oktṓpodes (ὀκτώποδες). If the word were native to Latin, it would be octōpēs ('eight-foot') and the plural octōpedes, analogous to centipedes and mīllipedes, as the plural form of pēs ('foot') is pedes. In modern Greek, it is called khtapódi (χταπόδι), gender neuter, with plural form khtapódia (χταπόδια).

In short, the answer to your question is yes; these words are in use and are mentioned in official sources only because the incorrect forms have seen such widespread use.


In my opinion, it is far better to use plurals in the English style. Though OCTOPUSES and OCTOPI are correct as the plural of OCTOPUS, we should give preference to OCTOPUSES.

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    But there's no language by which octopi should be correct, which is the point of the question; It should logically be either octopodes or octopuses. The question is why octopi is also allowed.
    – Jon Hanna
    Commented Feb 13, 2013 at 10:09

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