Definition
Equivocation is an informal fallacy in which a key term is used with one meaning in one premise of an argument and with a different meaning in another, allowing the conclusion to appear to follow when in fact the inference rests on two distinct concepts sharing a single word. The fallacy exploits lexical ambiguity: because logic operates on meanings rather than spellings, a single string of letters carrying two senses behaves logically as two different terms.
A textbook example: "The end of life is happiness. Death is the end of life. Therefore death is happiness." The phrase "end of life" means "purpose of living" in the first premise and "termination of living" in the second. Two propositions about different things have been juxtaposed as if they shared a subject.
Why it matters
How it works
The diagnostic move against suspected equivocation is straightforward: replace the ambiguous term throughout the argument with a fixed, more specific phrase that captures one of its meanings, and ask whether the argument still works. If the substitution succeeds in one premise but fails in another, the original argument depended on the term's ambiguity. The fallacy is then exposed; the substituted argument either becomes valid but contains a false premise, or remains invalid in form.
Equivocation often hides behind emotionally charged terms whose meanings drift in casual speech — "freedom," "natural," "rights," "fair." Such terms can be used to assert a relatively narrow technical claim in one breath and a broad evaluative claim in the next, with the audience supplying the conceptual bridge unconsciously. Insisting on a single working definition for the duration of an argument is the structural defense; it forces every premise to be true or false under the same interpretation, which is the minimum condition for valid inference.