principal

/ˈpɹɪnsəpəl/adjective

Ranking first in importance; main or chief.

Example sentence

“Tourism is the principal source of income for the island.”

Not 'principle', which is a rule or belief. The old classroom hook still works: the principal is your pal.

Related words

textbookA book that teaches a particular subject, written for students at schools or colleges.diplomaAn official certificate from a school or college confirming that a student has finished a course of study or earned a degree.chalkboardA dark board with a slate or enamel surface for writing on with chalk, used before whiteboards became common.curriculumThe full range of subjects and lessons taught at a school, college, or university.quizA short contest or test in which people answer questions.disciplineThe ability to control your own behavior; self-control.