Frequency: The word is one of the 3000 most common words in English.
Principal means first in order of importance.
The ~ reason for my change of mind is this.
...the country’s ~ source of foreign exchange earnings...
Their ~ concern is bound to be that of winning the next general election.
= main, chief
ADJ: ADJ n
The ~ of a school, or in Britain the ~ of a college, is the person in charge of the school or college.
Donald King is the ~ of Dartmouth High School.