Principal means first in order of importance.
The principal reason for my change of mind is this.
我改变主意的最重要原因就是这个。
the country's principal source of foreign exchange earnings.
该国最重要的外汇交易收入来源。
Their principal concern is bound to be that of winning the next general election.
他们最关心的肯定就是赢得下次大选。
principals
The principal of a school is the person in charge of the school or college.
Donald King is the principal of Dartmouth High School.
唐纳德·金是达特茅斯中学的校长。
The principal of a loan is the original amount of the loan, on which you pay interest.