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?
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?
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.
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?