is "modus operandi" singular or plural?

Is the phrase (as used in English) "modus operandi" singular or plural? And if the former, what is its plural form [or vice versa]?

(To my untutored eye, "modus" seems to be a singular form, while "operandi" seems to be a plural.)

A search of this site does not reveal anything bearing on the matter.

Solution 1:

Modus operandi is singular in both Latin and English. The plural is modi operandi, and, judging from this Ngram, I would advise against modus operandis.

Since there seems to be some confusion over why only modus changes form in the plural, but never operandi, I'll explain that too.

Modus is the most important word here, quite clearly meaning mode. It's a noun and nouns are simple. The plural is modi.

Operandi is rather more complex. We may translate it as of operation, but that's not very precise: mode of operation would be modus operationis. More exactly, operandi means of operating. It's a gerund, which can't be pluralized either in Latin or in English. However many times you've gone fishing, you'll still say that you like fishing, never that you like fishings. It's the same in Latin.

The confusion arises because the Latin gerund shares its singular forms with the gerundive, which can be pluralized. We know that modus operandi does not use the gerundive because then it would mean mode of something to be performed, which is rather nonsensical. So, I'd strongly advise against using modi operandorum, since I presume you don't mean modes of things to be performed - nor am I particularly sure what anyone might mean by it.

You might say that, if people used modus operandorum often enough, then using it would be fair game. You'd be right. Luckily for us, however, Ngrams doesn't have a single instance of it.

Solution 2:

It is singular. The plural form is modi operandi (modes of operation). See where it says plural in Merriam-Webster's, Dictionary.com's, and Wikipedia's entries.

Of note, modi operandorum (modes of operations) is never correct, as plural gerundiums are an impossibility in Latin[1][2].

Sources:

[1] See comments to this answer made by Cerberus, a scholar in the field.

[2] See this page on gerunds in Latin.

Solution 3:

It is a singular phrase. The plural form being "modi operandi".

You can find more about it at Merriam-Webster and TFD

• modus operandi - "A method of operating or functioning, a person's manner of working. TFD

Solution 4:

Modus operandi = the method of operating. (Plural: modi operandi , but NOT modi operandorum which is the genitive plural of the gerundive, not of the gerund. As others have said, you cannot pluralize a gerund in Latin. Otherwise in this case it would be of operatings, which is a nonsense.)

However: using the Latin noun for operation (operatio, operationis), as far as I know, you could have mode of operation, modes of operation, or mode of operations or modes of operations; each of the four being a slightly different concept. This could be achieved using the genitive of the noun operatio: that is genitive singular = operationis; and genitive plural operationem. Thus the four different possibilities would be modus operationis (mode of operation), modi operationis (modes of operation), modus operationem (mode of operations) and modi operationem (modes of operations). Does that make sense? But wouldn't it be far easier just to say it in English?