Tautology refers to redundancy, repetition, and 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.

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"?).