a.

1. 主要的，首要的，最重要的[B]
   Drinking is a principal cause of traffic accidents.

   酗酒是交通事故的主要原因。

2. 资本的，本金的

n.

1. 校长；社长；首长[C]
   The school principal read the honor roll list.

   校长宣读优秀生名单。

2. 资本，本金[S]
   The interest on a principal of $10,000 or above is 0.2 percent higher.

   本金一万元以上的利息高百分之零点二。

3. 主要演员，主角；首席演奏者；独唱者[P1]
4. 委托人，当事人[P1]

adj.

1. first in order of importance; main.
2. denoting an original sum of money invested or lent.

n.

1. the most important or senior person in an organization or group.
   ▸the head of a school or college.
   ▸(in certain professions) a fully qualified practitioner.
2. a sum of money lent or invested, on which interest is paid.
3. a person for whom another acts as an agent or representative.
   ▸Law  a person directly responsible for a crime.
4. a main rafter supporting purlins.
5. an organ stop sounding a set of pipes typically an octave above the diapason.