Other forms: begs the question; begging the question; begged the question
In rhetoric, to beg the question is to make an argument that assumes the truth of its conclusion, rather than proving it. When you say, "My dog's the best dog because he's the greatest," you beg the question.
Begging the question is also known as "circular reasoning," because in order for your argument to be true, the conclusion has to be true. In informal, everyday use, beg the question means "to raise a fairly obvious but still unanswered question." If your neighbor got hit by lightning on the roof of their house, it would beg the question: what were they doing on the roof in a thunderstorm? Though it's best avoided in formal contexts, this usage is the more common one.