Equivocation

The fallacy of equivocation is committed when someone uses the same word in different meanings in an argument, in such a way that the argument would be correct only if the word actually meant the same each time around.

For example, "A feather is light. What is light cannot be dark. So a feather can not be dark," commits this fallacy: The word light is used in the sense of having little weight the first time, but of having a bright colour the second time.