Circularity is not so much a logical fallacy but an ineptitude in arguing. It occurs when a series of premises depending on one assumption can only reach a conclusion by re-using the assumption as a proven premise. Below is an example:

(1) The New York Knicks are the best basketball team in the country.

(2) They are the best because of the shooting ability of Smith and Jones.

(3) Smith and Jones have great shooting ability since they play for the NY Knicks who, after all, are the best basketball team in the country.