The fallacy of equivocation is committed when someone uses the same word in different meanings in an argument, implying that the word means the same each time round.

For example:

A feather is light.

What is light cannot be dark.

So a feather cannot be dark.

The above argument commits this fallacy: The word light is used in the sense of having little weight the first time, but of having a bright colour the second time. Since the middle term in this syllogism is actually two different terms, equivocation is actually a kind of the fallacy of four terms.

The fallacy of equivocation is often used with words that have a strong emotional content and many meanings. These meanings often coincide within proper context, but the fallacious arguer does a semantic shift, slowly changing the context as he goes in such a way to achieve equivocation by equating distinct meanings of the word.

Equivocation is closely linked with the fallacy of amphiboly, where amphiboly relies on a syntactic shift.

Outside of its use as a technical term in logic, equivocation can also mean the use of language that is equally susceptible of being understood in two different ways, or, more generally, ambiguous language. There is often a connotation that the use is deliberate, and intended to deceive.