Tautology

tautology [taw-TALL-uh-jee]
n. a logical statement in which the conclusion is equivalent to the premise; circular reasoning.

[From Greek tautologos ("repeating what has been said") from tauto ("the same") + -logos ("saying")]

There are other definitions of tautology -- it can be a synonym for redundancy, for example -- but the circular argument usage is the one which interests me.

This is from Wikipedia and explains tautology well: "It is formulating a description in a way that masquerades as an explanation when the real reason for the phenomena cannot be independently derived. The statement 'If you can't find something (that you lost), you are not looking in the right place' is tautological. It is true and can't be disputed, but conveys no useful information. Any argument containing a tautology is flawed and must be considered a logical fallacy."

I like this example: "The reason I'm not wearing any socks is because I don't have any socks on."

Julian Baggini, writing in the London Guardian:

The seven wise men who are supposed to have come up with the wise words inscribed at Delphi come close to being just as vacuous. For the injunction not to do anything to excess is also a tautology: an excess just is too much, and so by definition is already wrong. The difficulty lies in knowing how much is too much, not that too much is bad.