When talking about terms it should be with article (a/an) before the term? For example:
1 What is an urticaria?
2 What is urticaria?
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It depends on the word you are asking about.
You would not use an article if you are asking about
"What is spaghetti?"
A proper noun, or a name.
"What is 'The New York Times'?"
A plural noun.
"What are fish?" or "What is a fish?"
In every other context, you would use an article.
As for the example you gave, "urticaria" is a mass noun, so you should say:
"What is urticaria?"