A circular argument is any argument in which the argument's conclusion is somehow included in its premises; in other words, the argument "proves" itself by assuming that it is correct.
- begging the question
- presuming the conclusion (or "implying the conclusion in the premise")
- the "No True Scotsman" fallacy
- argument by definition
"Only God can give us real truth. Therefore any arguments which say that real truth can come from scientific analysis must be wrong, because they aren't based on God's word. The more you think about this, the more you will see that all other ways to the truth are therefore dead-ends, which leaves only God's word as the path to real truth. QED."
"Redefining marriage to allow gay people to marry would be a bad idea because what's to stop us from redefining it again to something even worse?" The ending phrase presumes the conclusion that gay marriage is bad, with no supporting argument.
"Journalism requires journalists" (as part of an argument implying that bloggers are not "true" journalists because a journalist is someone who works for a print periodical or the news department of a radio or TV station) . The argument "proves" that X is not Y by (re)defining Y in such a way as to exclude X. It's also somewhat of the form "No true journalist...".