tautology

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Needless repetition of the same sense in different words; redundancy.
2. n. An instance of such repetition.
3. n. Logic An empty or vacuous statement composed of simpler statements in a fashion that makes it logically true whether the simpler statements are factually true or false; for example, the statement Either it will rain tomorrow or it will not rain tomorrow.

Century Dictionary and Cyclopedia

1. n. Repetition of the same word, or use of several words conveying the same idea, in the same immediate context. See dilogy.
2. n. The repetition of the same thing in different words; the useless repetition of the same idea or meaning: as, “they did it successively one after the other”; “both simultaneously made their appearance at one and the same time.” Tautology is repetition without addition of force or clearness, and is disguised by a change of wording; it differs from the repetition which is used for clearness, emphasis, or effect, and which may be either in the same or in different words.
3. n. Synonyms Redundancy, etc. See pleonasm.

Wiktionary

1. n. uncountable redundant use of words
2. n. countable An expression that features tautology.
3. n. countable, logic A statement that is true for all values of its variables

GNU Webster's 1913

1. n. (Rhet.) A repetition of the same meaning in different words; needless repetition of an idea in different words or phrases; a representation of anything as the cause, condition, or consequence of itself, as in the following lines: -- . The dawn is overcast, the morning lowers, And heavily in clouds brings on the day. Addison.

WordNet 3.0

1. n. (logic) a statement that is necessarily true
2. n. useless repetition