英汉词典

1. 首要的；最主要的；最重要的
    Principal means first in order of importance.

    e.g. The principal reason for my change of mind is this.
           这是我改变主意的最主要原因。
    e.g. ...the country's principal source of foreign exchange earnings...
           这个国家最主要的外汇收入来源

2. （中小学）校长；（英国）大学校长，学院院长
    The principal of a school, or in Britain the principal of a college, is the person in charge of the school or college.

    e.g. Donald King is the principal of Dartmouth High School.
           唐纳德·金是达特茅斯中学的校长。