Sentence:  Much of the controversy seems to revolve around ballot cards designed to be perforated by a stylus but which, either through error or deliberate omission, have not been perforated with sufficient precision for a machine to count them. In some cases a piece of the card—a <WSD>chad</WSD>—is hanging, say by two corners. In other cases there is no separation at all, just an indentation. 
Meaning of the word marked with <WSD> tags:  One of these pieces of paper.