- In Grammar and Rhetoric tautology is unnecessary repetition or the use of a word which adds nothing to the meaning of what is said. For example, to describe a man as 'an unmarried bachelor' is a tautology since if he is a bachelor, he must be unmarried: the word 'unmarried' is not needed and may be said to be otiose, pleonastic or redundant. The use of more words than are necessary to convey one's meaning - also known as pleonasm - is generally a fault of style, though it may sometimes be justified for the sake of emphasis, clarity, or explicitness, or for some other reason. The English word 'tautology' comes from the Greek ταυτολογἰα (tautologia), 'tautology', which in turn is a compound of το αυτο (to auto), 'the same', and λεγειν (legein), 'to say': there is a related verb ταυτολεγειν (tautolegein), 'to repeat what has been said'.
- In Logic a tautology is a statement which is always true, i.e., true whatever the circumstances, or a complex formula or statement which is true irrespective of the truth-value of its component formulae or statements. A simple example of a tautology in this sense is the statement 'Either it is raining or it is not raining': this statement is true if it is raining and true if it is not raining, and there is no third possibility.
There are two adjectives from 'tautology': 'tautological' - pronounced with the stress on the third syllable and a soft 'g', taw-ter-LO-dji-kel, IPA: /tɔː tə ˈlɒdʒ ɪk əl/ - and 'tautologous' - pronounced with the stress on the second syllable and a hard 'g', taw-TO-ler-ges, IPA: /tɔː tə ˈlɒg əs/.
Historico-philosophical note: It may seem strange, given the etymology of the word 'tautology', that logicians should have chosen to call statements like 'Either it is raining or it is not raining' tautologies: after all, they do not seem to involve unnecessary repetition. The explanation lies in the history of European philosophy in the nineteenth and early twentieth centuries. The German philosopher Immanuel Kant (1724-1804) used the word 'tautological' (or more accurately its German equivalent tautologisch) to characterize statements which are true simply in virtue of the meanings of the words used to express them, the clearest examples of such statements being definitions, such as 'A bachelor is an unmarried man'. Since in statements of this kind the second half is a repetition in different words of the first half, Kant's description of them as 'tautological' is readily understood. However, it is a conspicuous feature of Kant's tautologies that they do not convey information about the world but (merely) state rules of language: the statement that a bachelor is an unmarried man tells us how to use the English word 'bachelor' - it is not like telling us that John Smith is a bachelor or that there are more than a million bachelors in England. And it is this feature of Kant's tautologies, i.e., that they do not convey information about the world, that appears to be uppermost in the minds of later philosophers when they refer to statements which are always true as tautologies. See, for example, Ludwig Wittgenstein (1889-1951) Tractatus Logico-Philosophicus (1922), 4.46 ff., especially 4.462 'Tautologies ... are not pictures of reality', and 4.4611 'Tautologies ... are part of the symbolism'.