Equivocation is a logical fallacy resulting from the use of a single term for multiple meanings, e.g. A feather is light. What is light cannot be dark. Therefore, a feather cannot be dark. (Wikipedia)
Equivocation can be deployed as a rhetorical strategy, slipping between multiple definitions depending on which is useful in a given moment. Sometimes people call this the "motte and bailey" strategy, usually because they read Slate Star Codex.