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I often say

What is an idiom?

When I read Longman Pocket Idioms Dictionary Cased, I saw the sentence

What are idioms?

Are there any differences between the two forms? Which one is preferred?

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What are idioms? and What is an idiom? are both grammatical and both more or less express the same idea. The indefinite article can be used to refer to any member of the class of things described by the following noun and that is how it is used in What is an idiom? It’s a common enough usage. We might also say What is a poem? or What is a word?

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Purely my own opinion, but even though the two are (almost) identical in every context, I might be slightly more inclined to ask "What are idioms?" if I didn't have a clue, and wanted the quickest possible answer to give me a general idea. Whereas I might use "What is an idiom?" even if I already knew the general concept, but wanted to explore the definition in more detail. – FumbleFingers Jan 16 '12 at 17:36
@FumbleFingers: +1, because I suspect that's less an opinion and more a description of actual usage. – Barrie England Jan 16 '12 at 17:39
Per my comment to John's answer, I think he's nailed it anyway. But I'll leave my upvotes on both, just 'cos I can! :) – FumbleFingers Jan 16 '12 at 17:42

Idioms and an idiom in the two questions posed are both grammatical. They are both Generic Noun Phrases, which refer to classes and types of things, like the tiger in The tiger is endangered, which refers to the entire species.

Plural generic NPs (like idioms or tigers) and indefinite generic NPs (like an idiom or a tiger) are frequently encountered as predicate noun phrases, as in these two questions.

These questions do ask for slightly different answers, however. Indefinite generics are Definitions, so asking What is an idiom? is asking for a definition of the concept idiom. Plural generics, on the other hand, are about Norms, so asking What are idioms? can be interpreted as asking for a bunch of varied examples, perhaps with an added definition. You can make this sense clear by quantifying the plural generic noun with some: What are some idioms? unambiguously asks for a list.

In any event, it's the generic NPs that are important here, not the questions per se; generic Noun Phrases may appear in questions or statements of any kind, as may Generic Verb Phrases.

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Ah. I should have read your answer before commenting on Barries! I think I agree wwith you on the precise distinction which could exist - although as we're always saying here on ELU, context is everything. – FumbleFingers Jan 16 '12 at 17:38

What are Idioms is more correct when compared to what is an idiom? If a person raises a question to know the meaning of idioms, the more correct form is what are idioms? He will be talking about a number of idioms in general. And if a person raise a question after mentioning a particular idiom, he may ask what is an idiom?

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They are both equally correct. They are also very close to identical in meaning. – John Lawler Jan 16 '12 at 15:20

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