A lesson is a structured period of time where learning is intended to occur. It involves one or more students (also called pupils or learners in some circumstances) being taught by a teacher or instructor. A lesson may be either one section of a textbook (which, apart from the printed page, can also include multimedia) or, more frequently, a short period of time during which learners are taught about a particular subject or taught how to perform a particular activity. Lessons are generally taught in a classroom but may instead take place in a situated learning environment.
In a wider sense, a lesson is an insight gained by a learner into previously unfamiliar subject-matter. Such a lesson can be either planned or accidental, enjoyable or painful. The colloquial phrase “to teach someone a lesson”, means to punish or scold a person for a mistake they have made in order to ensure that they do not make the same mistake again.