Equivocation

In logic, equivocation ("calling two different things by the same name") is an informal fallacy resulting in the failure to define one's terms, or knowingly and deliberately using words in a different sense than the one the audience will understand.^{[1] [2] [3]}

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.^[1]

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.^[1]

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

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.

• The dictionary definition of equivocation at Wiktionary