Tautology (noun)

A statement or proposition that is true by definition, or a statement that is true by virtue of its logical structure.

Origin:

Derived from the greek words "tauto" (meaning "same") and "logos" (meaning "word" or "accoun.t")

Examples:

1. The statement "all bachelors are unmarried men" is a tautology.
2. The phrase "necessary necessity" is a tautology.
3. The argument that the earth is flat is a tautology.
4. The definition of a square as a rectangle with four equal sides is a tautology.
5. The statement "free gift" is a tautology, as something cannot be both free and not free.