Equivocation is a logical fallacy where a term changes its meaning in the course of an argument. Wikipedia gives the following example.
A feather is light.
What is light cannot be dark.
Therefore, a feather cannot be dark.
We all know that for example a crow’s feather is dark. In this case “light” changes its meaning. First "light" means, “Not heavy”. Later “light” means "bright."
- Equivocation (Wikipedia article)