Some English words borrowed directly from other languages retain the plural form of the other language.
For example, fermata is pluralized as fermate , and cirrhosis becomes cirrhoses .
The most familiar examples are words taken directly from Latin, of which there are at least hundreds: persona / personae , matrix / matrices , fulcrum / fulcra , and so forth, but there are examples from Greek, Italian, and so on.
Over time, some plurals have been regularized, especially in American English, and especially when referring to modern concepts as opposed to ancient— there are far more database schemas than database schemata out in the wild.
Because the Latin irregular plurals are most common, and sound loftier than the regular plural, there is a tendency to hypercorrect the plural of any word ending in -us and -um
in particular, even when there is no historical basis for it.
And thus, Oxford tells us The standard plural in English of octopus is octopuses .
However, the word octopus comes from Greek, and the Greek plural form octopodes is still occasionally used.
The plural form octopi is mistakenly formed according to rules for Latin plurals, and is therefore incorrect.
At some point, of course, any "incorrect" usage that becomes popular enough becomes "correct," and so it is that various dictionaries will include octopi (and ethoi and diplomae and so on) as acceptable plurals.
American Heritage and Merriam-Webster list it as an acceptable alternative.
M-W has an Ask the Editor segment on the matter.
ell.stackexchange.com/users/21/choster
As snailplane pointed out in the comments, octopus is derived from Greek, and so the correct plural would be octopodes , not octopi .
From Oxford Dictionaries.com : The standard plural in English of octopus is octopuses .
However, the word octopus comes from Greek and the Greek plural form octopodes is still occasionally used.
The plural form octopi , formed according to rules for some Latin plurals, is incorrect.
And from the Online Etymology Dictionary : octopus (n.) 1758, genus name of a type of eight-armed cephalopod mollusks, from Greek oktopous , literally "eight-footed," from okto "eight" (see eight)
+ pous "foot" (see foot (n.)).
Proper plural is octopodes , though octopuses probably works better in English.
Octopi is from [the] mistaken assumption that -us in this word is the Latin noun ending that takes -i in plural.
As pointed out above, octopuses would probably be acceptable in all but the most formal of circumstances.
ell.stackexchange.com/users/3976/starsplusplus
From The Corpus of Contemporary American English (COCA) and The British National Corpus (BNC) : COCA BNC octopuses 147 results 29 results octopi 44 results 11 results octopoda 4 results 0 results octopodes 2 results 3 results The most common plural is octopuses .
Use that.
The alternative octopi is also somewhat common.
In fact, it's common enough that we should probably consider it standard as well, but because octopus isn't actually derived from Latin , pluralizing it this way may rankle some feathers.
If you'd like to avoid criticism, avoid using this form.
We'll discount octopoda entirely since all four results are from Orson Scott Card's Lost Boys , in which fictional characters discuss the proper plural of octopus .
The least common on the list is octopodes .
Although you can make an argument that this plural is more correct than octopi , the fact is few people know it and fewer people use it.
If you pluralize octopus this way, you'll draw more attention to how you're saying it than what you're saying.
Octopodes probably isn't common enough to be considered standard, and worse, it may fail to communicate entirely.
The verdict?
If you want to communicate, use octopuses .
Save the other plurals for when you want to use words like toys.
ell.stackexchange.com/users/230/snailplane
Even one source will change its answer to a question such as this as time passes and language evolves.
For example: The 1998 edition of Chambers Dictionary gives octopuses as the main answer, and lists octopodes as an archaic form.
I know that earlier editions of the Chambers Twentieth Century Dictionary did not have the 'archaic' annotation.
Their web site currently only lists octopuses .
As to the red line to which you refer, I shall assume that it was added by your word processor's spell check function.
That is, of course, dependent upon the dictionary which it uses and, as can be seen from the other answers, they don't all entirely agree, though almost all of them agree that octopi is wrong, for the reasons explained by choster.
ell.stackexchange.com/users/5877/ClickRick
"octopuses" and "octopi" are valid plurals for "octopus".
I prefer "octopi" --
it sounds more entertaining, without risking a double entendre .
Spell-checkers are often incorrect.
For example, good spell checkers do not include the word "calender", because misspellings of "calendar" are far more common than correct usages of "calender".
Also, most spell-checkers are implemented by building a list of common base words, and adding some rules for common variations on those base words.
( Programming Pearls has an essay on how combining this approach with "hashtables" allows spell-checking large numbers of words, while using surprisingly little memory.)
This means that every unusual ending needs to be specially added to the dictionary.
For example, the dictionary could include "octopus" and have a rule that "octopus" + "es" is also a valid word.
If nobody bothered to add "octopi" to the dictionary, it would be marked in red by your spell-checker.
By the way, StackExchange's spell-checker complains about "StackExchange" and "hashtable", but not about "octopuses" or "octopi".
ell.stackexchange.com/users/9100/Jasper
What is the plural of octopus? Is it octopuses, octopi or octopodes?  Normally in English it's supposed to be octopi, but when I type it, it has a red line under it (by the spell check), which means that it's incorrect. I also saw the word octopuses in my school's textbook. I got really confused about which one is correct.  Thanks! Any help would be appreciated.
What is the plural of octopus?
ell.stackexchange.com
<plural-forms><latin>
ell.stackexchange.com/questions/24655
ell.stackexchange.com/users/6832/ʇolɐǝz ǝɥʇ qoq
The most common plural form is octopuses. As it is a Greek word, the correct plural is octopodes. However, octopi is also used. Octopuses and octopodes can vbe used, octopi is also suggested.
Octopuses and octopodes can vbe used, octopi is also suggested.