A lesson is the combination of the elements class , teacher , subject and room with a specific number of periods and perhaps with additional parameters. A difference is made between planned and scheduled lessons.
A planned lesson would be e.g. that teacher Callas should teach two periods of music to class 1a in room R1a.
The scheduled lesson would also contain the position of the lesson in the timetable, e.g. the lesson takes place on Mo-2 and Th-1.