Equivocation

In logic, equivocation 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.
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 produces a fallacy of four terms. Below is an example:
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.