tau·tol·o·gy / tôˈtäləjē/ • n. (pl. -gies) the saying of the same thing twice in different words, generally considered to be a fault of style (e.g., they arrived one after the other in succession). ∎  a phrase or expression in which the same thing is said twice in different words. ∎  Logic a statement that is true by necessity or by virtue of its logical form.DERIVATIVES: tau·to·log·i·cal / ˌtôtlˈäjikəl/ adj.tau·to·log·i·cal·ly / ˌtôtlˈäjik(ə)lē/ adv.tau·tol·o·gist / -jist/ n.tau·tol·o·gize / -ˌjīz/ v.tau·tol·o·gous / -gəs/ adj.

TAUTOLOGY, also PLEONASM. A term in RHETORIC for unnecessary and ineffective REPETITION, usually with words that add nothing new: She was alone by herself. Many tautological (or tautologous) expressions occur in everyday usage. The tautology in some is immediately apparent: all well and good; cool, calm, and collected; free, gratis, and for nothing. In others, it is less obvious, because they contain archaic elements: by hook or by crook; a hue and cry; not a jot or tittle; null and void; rack and ruin. Compare CIRCUMLOCUTION, REDUNDANCY.

tautology The use of words to repeat (unnecessarily) the same statement or meaning. For example, the statement that ‘Britain is an island and surrounded by water’ is a tautology, since islands are by definition so described. Tautological explanations are similarly true by definition, or circular, and therefore unfalsifiable. Sociological explanations which locate the origins of social institutions in their effects tend to take this form. Thus, for example, some early functionalist anthropologists (including Bronislaw Malinowski) were prone to argue that, because certain (exotic) social practices (such as witchcraft) existed, then they must have a social function—and that one could assume they had that function precisely because the practices themselves existed.

tautology A law of logic, in the form of a proposition, that describes a universal truth; no matter what values are assigned to the variables in the proposition the result is always true. An example from the propositional calculus is (P ∨ Q)′ = P′ ∧ Q′

where ∨ and ∧ are the or and and operators and P′ is the negation of P. In the truth table for a tautology the final result column contains only the value true. If the final column contains only the value false, then a contradiction has been identified.

tautology XVI. — late L. tautologia — Gr. tautologíā, f. tautológos repeating what has been said (whence tautologous XVIII), f. tautó the same + -logos saying; see -LOGY.