Tautology is used so often in articles and essays I read, I figured that I should know the proper definition.

taw-tol-uh-jee /tɔˈtɒl ə dʒi/

From Wikipedia: Tautology (from Greek tauto, “the same” and logos, “word/idea”) is an unnecessary repetition of meaning, using dissimilar words that effectively say the same thing (often originally from different languages). It is considered a fault of style and was defined by A Dictionary of Modern English Usage (Fowler) as “saying the same thing twice,” if it is not apparently necessary for the entire meaning of a phrase to be repeated. If a part of the meaning is repeated in such a way that it appears as unintentional, or clumsy, then it may be described as tautology. On the other hand, a repetition of meaning which improves the style of a piece of speech or writing is not necessarily described as tautology.

For fun, see also antanaclasis.