Blasphemy is the act of speaking about God or sacred things in a deeply disrespectful way. In many religions, blasphemy is considered a serious offense. During the Middle Ages, people could be punished by law for blasphemy, and some countries still have laws against it today.

The word can also be used in a looser, sometimes humorous way to describe any remark that offends something people care deeply about: “My uncle says that putting ketchup on a hot dog is blasphemy.”