keysubject
nounUK /ˈkiːˌsʌbdʒɪkt/
US /ˈkiˌsʌbdʒɛkt/
The main thing a story or lesson is about.
The key subject of today's science class was how plants grow.
OTHER MEANINGS
2. The most important person or thing being studied or discussed.
The key subject of the meeting was the new playground.
3. A required or essential course in school.
Math is a key subject for every student.
4. The main person in a painting or photograph.
The key subject of the photo was a smiling girl.