Equivocation is a logical fallacy that involves taking a word with more than one definition and freely substituting one definition for another.
For example: "A feather is light. Therefore, a feather cannot be dark." There are two meanings of the word "light." The first sentence assumes a meaning that is the opposite of "heavy," not the opposite of "dark."
This fallacy is used frequently in the service of apologetics arguments. A few examples:
- Atheism is based on faith. There are multiple meanings of the word "faith".
- No true Scotsman fallacy. When somebody says "So-and-so wasn't really a Christian because he did that," they are relying on ambiguity in the word "Christian".