Equivocation Fallacy

It is easy to assume this one is invoked because someone knows their argument doesn't hold up otherwise, but I suspect that it is usually made through honestly muddled thinking combined with a mistaken notion of cleverness.

Equivocation Fallacy
Other name(s): Doublespeak
Type:  Informal
Category: Ambiguity

Definition
Using more than one meaning of a key word or phrase in an argument with an implication that they are the same.

Usual Form

Statement is made using one meaning or sense of a concept, implying (or sometimes stating) a different meaning in its conclusion.
Often takes the form of a syllogism.

Explanation

For an argument to work, words must have the same meaning each time they appear in its premises or conclusion. Equivocation occurs when arguments switch between different meanings of words. A change in meaning introduces a change in subject, and invalidates the argument because the premise(s) and conclusion now refer to different things.

It can be tricky to notice that a change has taken place, especially if one has become used to confusing the two meanings. Bringing up the fact that there are two meanings involved will generally invoke cognitive dissonance and all the reactions one expects from exposing such things to light.

"To expose the fallacy of equivocation you give accurate and specific definitions of terms, and show carefully that in one place the definition of the terms was different from the definition in another."
(Robert Huber and Alfred Snider, Influencing Through Argument. IDEA, 2005)

Examples

"(1) Jesus is the Word of God.
(2) The Bible is the Word of God.
Therefore:
(3) Jesus is the Bible."
The phrase “Word of God” means very different things in the two premises, so this argument rests on an equivocation, in the form of a bad syllogism.

"The sign said 'fine for parking here,' and since it was fine, I parked there."
When the two meanings are widely understood as quite different, equivocation can be a source of humor.

"You should not use the term 'illegal alien' because they're people, and people cannot be illegal."
"You are not fat. You have fat. You have fingernails but you are not fingernails."
This fallacy is very, very popular for political and feelgood propaganda.