Definition of "tautology" [tau•tol•o•gy]

• 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. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "This is the logical notion of tautology, which is very different from the way the term tautology is used in stylistics"
• "Digital download? isnt that what they call a tautology?"
• "It's only a tautology is you pretend life and niches don't have distinguishable properties and that a niche can't be populated."