tautology

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Needless repetition of the same sense in different words; redundancy.
• noun An instance of such repetition.
• noun 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.

from The Century Dictionary.

• noun Repetition of the same word, or use of several words conveying the same idea, in the same immediate context. See dilogy.
• noun 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.”
• noun Synonyms Redundancy, etc. See pleonasm.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable redundant use of words
• noun countable An expression that features tautology.
• noun countable, logic A statement that is true for all values of its variables

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (logic) a statement that is necessarily true
• noun useless repetition