A tautology is a statement which is true by its own definition, and is therefore fundamentally uninformative. Tautologies use circular reasoning within an argument or statement.
In logic, a tautology is a statement that is true regardless of the truth-values of its parts. For example, the statement "All crows are either black, or they are not black," is a tautology, because it is true no matter what color crows are. As a humorous example, the tautology is famously defined as "that which is tautological". (That definition is, of course, tautological.) In a more realistic example, if a biologist were to define "fit" in the phrase "survival of the fittest" as "more likely to survive", he would be forming a tautology.
The opposite of a tautology is a contradiction, which is a statement that is always false.
In linguistics, a tautology is often a fault of style. It was defined by Fowler as "saying the same thing twice". For example, "three-part trilogy" is tautologous because a trilogy, by definition, has three parts. "Significant milestone" and "significant landmark" are also if less obviously tautologous, because milestones and landmarks are again significant by definition (could one imagine an "insignificant landmark"?). Sometimes a tautology can amplify the meaning, or it can invert the meaning. "Really existing socialism" - means: there is no socialism at all.
