Is one that goes unasked. At least, that's what I was told in junior high and high school.
Yet I avoided asking questions, because I was afraid of asking the dumb one. So if I didn't get something, I just let it simmer and either come to me on its own or wait until I forgot all about the item, whichever came first. Usually it was the latter. On the rare occasions that I did muster the courage to ask a question about a concept I didn't understand, I'd get a look from the teacher like I was the biggest idiot ever to walk erect and a dismissive explanation that used the same words in a slightly different order. So I continued to shut up and color.
This habit has continued with me through my adult life, and I'm still never comfortable asking questions, which is usually to my detriment, especially as a law student. I am a bit better, though, than I was in the old days.
Now, about the statement that there's no such thing as a bad question. I can assure you that is not the case. I taught for a few years and have heard many different questions. Most of them were quite good. Even the questions that weren't on point but the student was actually trying to learn something were all right. However, I encountered my fair share of dumb questions. Here's how I define a dumb question:
A question asked not for the acquisition of knowledge, but rather as a means of procrastinating or being difficult.
These questions are usually asked by people who contain a certain degree of smug and inflated sense of self-worth. The person often feels put off by the fact that his time is being wasted learning this stuff that he signed up to learn. I hated those questions, and have vowed never to become "that person."
So anyway, moral of the story - there is such a thing as a dumb question.