The fallacy of equivocation occurs when an arguer relies on a key term to make an argument, but is inconsistent in how that term is defined.

Here is an example:

A State senator said of her proposed Bill:

"I have agreed that expensive Bills -- and I mean those that end up costing more than $100 million -- should not be passed when our State budget is running a large deficit, like it is now. But my proposed Bill will only call for bonds of $50 million, at 9% interest over ten years, and I don't define that as an expensive bill."

The problem here is that the proposed bill, with interest payments included, will end up costing the State around $120 million -- so under the definition provided in the first sentence, it would be an "expensive bill." But in the second sentence, the speaker changes the definition of "expensive," saying that she would not "define that as an expensive bill." This means the speaker has changed definitions of a key term that she relies upon to make her argument.

Equivocation also applies to cases wherein a speaker departs from the broadly accepted definition of a word, without making it explicit that they are using their own definition.

Equivocation is often combined with another fallacy, such as quotation out of context, or the fourth term fallacy.