"Begging the question" is the use of a proposition as a premise in
an argument intended to support that same proposition. This is
fallacious and should not be permitted in reasoning. The
point of reasoning is to throw light on the truth or falsity of a
proposition (the conclusion) by relating it to other propositions
(the premises) that we already have some basis for believing to
be true. If our reasoning does nothing more than relate p to
itself, then we haven't gained anything.
The most obvious way to commit the fallacy would be simply to
restate the conclusion as a premise.
A more subtle form of the fallacy occurs when the circle is
enlarged to include more than one step; the conclusion p is
supported by premise q, which in turn is supported by p (though
there could be any number of intervening steps).
Another common form of begging the question is known as the
complex question. In questions of this type, one attempts to get someone to accept a proposition by putting forth a question that presupposes it.