Fetching details from the dictionary...

lesson
section (in an organization); division; department
counter for lessons and chapters (of a book)
今日は教科書の第五課を勉強します。
Today we will study lesson five.
この課の内容は難しいですね。
The content of this lesson is difficult.
次の課に進む前に復習しましょう。
Let's review before moving to the next lesson.