Equivocation is a fallacy of ambiguity in which a word changes meaning within an argument. For instance, "The sign says fine for parking. Since it is fine, I can park here." With this example the word fine changes meanings from a sum of money extracted as a penalty by an authority to something which is satisfactory or acceptable.
Here are two more examples: "A feather is light. What is light cannot be dark. Therefore a feather cannot be dark." and "Only man is logical. No woman is man. Therefore, no woman is logical."