A circular argument is one in which "proof or evidence involving premises which assume the conclusion which is to be established."
It is not a matter of logical validity here, but more an "informal" fallacy
in an argument. Usually circular arguments are very complex and not at all immediately obvious because of the use of synonyms in the terms (another informal fallacy called "accent" rampant on these forums) that hide the fallacy. A common circular argument is that the Bible must be true because it is the word of God [and God would not lie to us]; this turns out to be a tautology: the Bible is the Word of God because the Bible is the Word of God.
Here is another example (suggested by Copi, Intoduction to Logic
. 2nd edition, 1961):
One argues that Shakespeare is a greater writer than Ayn Rand because people with Good Taste in literature prefer him. If asked how one determines who has Good Taste in literature, one replies that they are identified by preferring Shakespeare to Rand.
Circular arguments are a form of petitio principii, or 'begging the question.'