节(jié) - measure word for lessons, classes or sections in Mandarin

节 (jié) is the measure word used for lessons, classes and sections, as in 一节课 (yì jié kè), "one lesson" or "one period". It is the same character that appears alone in words like 春节 (Chūnjié), Chinese New Year, and 节日 (jiérì), "festival", where it carries the sense of a "joint, segment, node" (think of the segments of bamboo), extended here to mean a discrete segment of the school day.

Always pair it with 课 (kè), "class, lesson": 两节课 (liǎng jié kè) is "two lessons", not 两个课 (liǎng gè kè), which sounds wrong to a native speaker even though 个 (gè) is the safe default measure word almost everywhere else.

节 (jié) also measures other segmented things: 一节电池 (yì jié diànchí), "one battery", or 一节车厢 (yì jié chēxiāng), "one train carriage", so once you have the "segment of something longer" logic, the same word keeps reappearing well beyond the classroom.