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Etymology and pluralisation

The scientific Latin term octopus was derived from Ancient Greek ὀκτώπους (oktōpous), a compound form of ὀκτώ (oktō, 'eight') and πούς (pous, 'foot'), itself a variant form of ὀκτάπους, a word used for example by Alexander of Tralles (c. 525 – c. 605) for the common octopus. The standard pluralised form of octopus in English is octopuses; the Ancient Greek plural ὀκτώποδες, octopodes (/ ɒ k ˈ t ɒ p ə d iː z /), has also been used historically. The alternative plural octopi is usually considered grammatically incorrect because it wrongly assumes that octopus is a Latin second-declension -us noun or adjective when, in either Greek or Latin, it is a third-declension noun.

Historically, the first plural to commonly appear in English language sources, in the early 19th century, is the latinate form octopi, followed by the English form octopuses in the latter half of the same century. The Hellenic plural is roughly contemporary in usage, although it is also the rarest.

Fowler's Modern English Usage states that the only acceptable plural in English is octopuses, that octopi is misconceived, and octopodes pedantic; the last is nonetheless used frequently enough to be acknowledged by the descriptivist Merriam-Webster 11th Collegiate Dictionary and Webster's New World College Dictionary. The Oxford English Dictionary lists octopuses, octopi, and octopodes, in that order, reflecting frequency of use, calling octopodes rare and noting that octopi is based on a misunderstanding. The New Oxford American Dictionary (3rd Edition, 2010) lists octopuses as the only acceptable pluralisation, and indicates that octopodes is still occasionally used, but that octopi is incorrect.