Equivocation

In logic, equivocation ("calling two different things by the same name") is an informal fallacy resulting from the use of a particular word/expression in multiple senses within an argument.

It is a type of ambiguity that stems from a phrase having two or more distinct meanings, not from the grammar or structure of the sentence.

Fallacy of four terms
Equivocation in a syllogism (a chain of reasoning) produces a fallacy of four terms (quaternio terminorum). Below is an example:


 * Since only man [human] is rational.
 * And no woman is a man [male].
 * Therefore, no woman is rational.

The first instance of "man" implies the entire human species, while the second implies just those who are male.

Motte-and-bailey fallacy
Equivocation can also be used to conflate two positions which share similarities, one modest and easy to defend and one much more controversial. The arguer advances the controversial position, but when challenged, they insist that they are only advancing the more modest position.