Is a good that when consumed yields disutility i.e consumption is unpleasant or does not lead to satisfaction or positive utility. For example if a consumer consumed expired food that makes you ill, this would be an example of a bad good as after consumption the consumer will have less utility than before. In effect these goods ar just opposites of a normal good.
However, there is some subjective judgement over whether a good is beneficial to consume or in fact it leads to reduced utility. For instance, even though the health effects of smoking can make smoking a bad, smokers believe they enjoy smoking which make it a good at the time of consumption.