A lesson is a unit in teaching, typically involving one particular thing to be taught or practiced. At most schools around the world, a lesson is of a specific duration such as 45, 60 or 90 minutes, and devoted to a specific school subject (e.g. reading, writing or mathematics). The weekly lessons follow a timetable also known as a class schedule. Teachers and instructors usually have a lesson plan which dictates the structure of the teaching.
The term lesson can also refer to something taught outside school. For example, when someone has committed an offense and is punished for it, people hope that he or she has "learned his lesson", i.e. has learned not to repeat the offense. In this sense, giving a punishment is sometimes referred to as "teaching someone a lesson".