I think that questions titled "what is X?" is a broad enough category that they deserve a treatment similar to that of homework questions: the goal should not be to establish whether they should be allowed or not, but rather to agree on how they can be asked in an acceptable manner.
If a question is something small and simple, with a definite answer, like "what does this symbol mean?" I see no harm in answering it. The one previously asked received helpful, informative answers, and now anyone with the same question will be able to find it with a search without wading through pages of porn.
If a question is somewhat open-ended, but the asker gives enough background and details to establish exactly what he/she is looking for, that's also fine. If such details are missing, ask for them in a comment. When they get added, answer the question. This question on well-behaved functions is an example of that process working successfully.
This question on limits is missing that last step. As a result, it's not really clear what a good or correct answer would be. I will try to write my own comprehensive answer to the question this weekend, aimed at a typical college freshman, simply because I think it is important to have such an answer on this site to be pointed to in the future. Even so, I have no way of knowing whether I am really giving the question answerer what he is looking for.
In fact, I might be in favor of allowing these types of questions for such fundamental concepts like 'limits' or 'proofs' in the beginning, so that in the future, there will be an annotated list of high-quality answers that can be used as a reference.
A question that is clearly a joke or is impossible to answer deserves to be closed promptly.