Equivocation

Equivocation is a fallacy by which an important word or phrase in an argument is used with more than one meaning, or using an ambiguous term in more than one sense, thus making an argument misleading. It is also known as semantic equivocation. 
Example: “Micah White says he has endured 'persecution' for his beliefs, but an atheist is, by definition, one who lacks beliefs.” (This equivocation implies that Micah White can’t be persecuted for his beliefs because atheists lack beliefs.)
(Letter to New York Times by Michael Scheer published in 1999)