1. Ideas or words which are used when speaking or writing that do not have any rational meanings: Some reporters write a lot of nonsense which seems to appeal to many people.

Many of the words that are presented in some dictionaries are nonsense because they often use another form of the word that is being defined to present it. For example, "photosensitization": "The process of photosensitizing."

2. Behavior which is annoying, silly, unkind, or ridiculous: The teacher, Mrs. Jims, was not in the mood to put up with any nonsense from the students who were listening to music from their cell phones instead of doing their assignment.