Plural form of words ending in -us
In English, the plural form of words ending in -us, especially those derived from Latin, often replaces -us with -i. There are many exceptions, some because the word does not derive from Latin, and others due to custom. Conversely, some non-Latin words ending in -us and Latin words that did not have their Latin plurals with -i form their English plurals with -i. Some words' plurals end in -i even though they are not Latin, or that is not the Latin plural, e.g., octopi is sometimes used as a plural for octopus. Prescriptivists consider these forms incorrect, but descriptivists may simply describe them as a natural evolution of language.
Some English words of Latin origin do not commonly take the Latin plural, but rather the regular English plurals in -s: campus, bonus, and anus; while others regularly use the Latin forms: radius and alumnus. Still others may use either: corpus, formula, index.
History
In Latin, most second declension masculine nouns in -us form their plural in -i. However, some Latin nouns ending in -us are not second declension. For example, third declension neuter nouns such as opus and corpus have plurals opera and corpora, and fourth declension masculine and feminine nouns such as sinus and tribus have plurals sinūs and tribūs.Some English words derive from Latin idiosyncratically. For example, bus is a shortened form of omnibus 'for everyone', the ablative plural of omnis, and ignoramus is a verb form, 'we do not know'. Syllabus is a Late Latin word, derived from a misreading of the Greek sittybos "table of contents"; since it is not a classical word, it does not have a classical plural.
''Virus''
The English plural of virus is viruses. In most speaking communities, this is non-controversial and speakers would not attempt to use the non-standard plural in -i. However, in computer enthusiast circles in the late 20th century and early 21st, the non-standard viri form was well attested, generally in the context of computer viruses. Viri is also found in some nineteenth-century sources.While the number of users employing these non-standard plural forms of virus was always a small percentage of the English-speaking population, the variation was notable because it coincided with the growth of the web, a medium on which users of viri were over-represented. As the distribution of Internet users shifted to be more representative of the population as a whole during the 2000s, the non-standard forms saw decline in usage. A tendency towards prescriptivism in the computer enthusiast community, combined with the growing awareness that viri and virii are not etymologically supported plural forms, also played a part.
Nonetheless, the question of what the Latin plural of virus would have been in ancient times turns out not to be straightforward, as no plural form is attested in ancient Latin literature. Furthermore, its status as a second declension neuter noun ending in -us and not of Greek origin obscures its morphology, making guesses about how it should have been declined difficult.
Mass noun in Latin
The Latin word vīrus means "1. slimy liquid, slime; 2. poison, venom", denoting the venom of a snake. This Latin word is probably related to the Greek ἰός meaning "venom" or "rust" and the Sanskrit word viṣam meaning "toxic, poison".Since vīrus in antiquity denoted something uncountable, it was a mass noun. Mass nouns pluralize only under special circumstances, hence the non-existence of plural forms in the texts.
There is no known plural for this word in Classical Latin. It is unclear how a plural might have been formed under Latin grammar in ancient times if the word had acquired a meaning requiring a plural form. In Latin, vīrus is generally regarded as a neuter of the second declension, but neuter second declension nouns ending in -us are rare enough that inferring rules is difficult. Plural neuter nouns of other declensions always end in -a.
In Neo-Latin, a plural form is necessary in order to express the modern concept of ‘viruses’, which lead to the following declension:
| singular | plural | |
| nominative vocative accusative | vīrus | vīra |
| genitive | vīrī | vīrōrum |
| dative ablative | vīrō | vīrīs |
Treating ''vīrus'' as 2nd declension masculine
If vīrus were a masculine second declension term like alumnus, it would be correct to use vīrī as its plural. However, it is neuter.There does exist a Latin word virī, meaning "men", but it has a short i in the first syllable.
The form vīriī is impossible as a plural of vīrus, since we only find the ending -iī in the plural form of masculine and feminine words ending in -ius. For instance, radius is pluralized by removing -us, to isolate the stem radi-, and then adding the plural suffix -ī. Thus the -iī ending of the resulting word radiī is not a suffix: it is simply the consequence of adding the actual suffix ī to a stem that has an i as its last letter. Vīriī would be the plural form of the putative, nonexistent word vīrius.
The form viruses appears in the official Scrabble words list, but neither viri nor virii does. Similarly, the spellchecker built into the Mozilla Firefox browser accepts viruses but neither viri nor virii.
Use of the form ''virii''
Usage of virii within Internet communities has met with some resistance, most notably by Tom Christiansen, a figure in the Perl community, who researched the issue and wrote what eventually became referred to in various online discussions as the authoritative essay on the subject, favoring viruses instead of virii. The impetus of this discussion was the potential irony that the use of virii could be construed as a claim of superior knowledge of language when in fact more detailed research finds the native viruses is actually more appropriate. In other words, virii is a hypercorrection.Biology
In life sciences, "viruses" generally refers to several distinct strains or species of virus. "Virus" is used in the original way as an uncountable mass noun, e.g. "a vial of virus". Individual, physical particles are called "virions" or "virus particles".''Octopus''
There are three plural forms of octopus: ', ', and . A fourth form octopods is occasionally used by scientists for taxonomic purposes.Currently, octopuses is the most common form in the UK as well as the US; octopodes is rare, and octopi is often objected to as incorrect.
The Oxford English Dictionary lists octopi, octopuses and octopodes ; it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun. Rather, it is Ancient Greek, from oktṓpous, whose plural is oktṓpodes. If the word were native to Latin, it would be octōpēs and the plural octōpedēs, analogous to centipedes and mīllipedēs, as the plural form of pēs is pedēs. In modern Greek, it is called khtapódi, with plural form khtapódia.
Chambers 21st Century Dictionary and the Compact Oxford Dictionary list only octopuses, although the latter notes that octopodes is "still occasionally used"; the British National Corpus has 29 instances of octopuses, 11 of octopi and 4 of octopodes. Merriam-Webster 11th Collegiate Dictionary lists octopuses and octopi, in that order; Webster's New World College Dictionary lists octopuses, octopi and octopodes.
A search on PubMed for titles or abstracts of peer-reviewed scientific papers found
octopuses in 176 papers, octopi in 15 papers, and no records for octopodes. However, octopods was found in 53 papers. When expanding the search to all fields, found octopods, octopi, and octopodes all stayed the same, but octopuses increased to 1054 papers. Indicating octopuses is the most preferred in the scientific record, whereas octopodes is never used.
Fowler's Modern English Usage states that "the only acceptable plural in English is octopuses," and that octopi is misconceived and octopodes pedantic.
The term octopod is taken from the taxonomic order Octopoda but has no classical equivalent. The collective form octopus is usually reserved for animals consumed for food.