What is the meaning of "personnel"? What is its plural form?

Solution 1:

In your definition no. 2, personnel is used as short for personnel department, i.e. the department of a company responsible for hiring and firing personnel. It is just like research standing for research department, etc.

Better take those papers to Research. Or, wait, I think the guys at Personnel should have a look at it first, since it concerns hiring new research personnel.

I consider the word personnel to be singular. The plural personnels is not used, because this word is a collective noun, i.e. a singular noun that can be treated as plural and gets a plural verb, just like police, family, etc. Whether to use a singular or plural verb depends on whether you are thinking of the group of people that make up the personnel (plural), or rather the concept or unit of personnel (singular, less frequent).

• The personnel have been complaining about working late.

• Our well trained personnel is our greatest asset. (Plural might be possible too, but probably less common in this sense.)

• I think Personnel is not going to be happy about moving to a new building. (The Personnel Department; plural would probably work as well? Not sure.)

Note that plural verbs with collective nouns are more frequent in England than in America.

[ Some people would call collective nouns plural, but I'm not a big fan of that. I consider plural primarily an inflectional quality, its syntactic ramifications being secondary. That is how plurals are analysed in other languages as well. Just as we only call committees plural, never committee, not even in sentences like the committee have come to a decision. But I can understand if some people might disagree. Each model has its merits. ]

Solution 2:

I would have said that personnel is plural, and has no singular form. Some of the dictionaries disagree with me (although from Robusto's answer, NOAD agrees with me), but consider the google Ngram:

It looks like personnel used to be singular in the 1920's, but is now trending towards being only a plural noun. Here are some excerpts from Google books where personnel is treated as singular:

This personnel is supplied only on the request of the State health authorities ... (1918)
That the personnel of a formation or establishment is armed and uses its arms in self-defence or in defence of its sick or wounded. (1917)
The personnel is divided into three classes: (1915)

As Bogdan Lataianu remarks in a comment, none of the results in this Google Ngram individually establishes anything, since you have constructions like "The shortages of personnel are apparent ...", where both is and are can appear after personnel for an unrelated reason. However, I think the general trend of the graph from personnel is to personnel are can only be explained by personnel moving from a singular noun to a plural one, at least in American English. British English handles verb agreement with collective nouns quite differently; see Cerberus's answer.

Solution 3:

First, the plural of personnel is personnel. It is a plural noun:

From NOAD:

personnel |ˌpərsəˈnel| plural noun people employed in an organization or engaged in an organized undertaking such as military service compare with materiel : : many of the personnel involved require training | sales personnel.

It just means the people of an organization.