a US : the person in charge of a public school
the new high school principal
b British : the person in charge of a university or college
2 
: an amount of money that is put in a bank or lent to someone and that can earn interest — usually singular
Our monthly mortgage payment covers the interest on our loan as well as some of the principal.
3 
a : the main performer in a group of performers
The ballet featured the two principals. [=lead dancers]
b : the most important person in a group
One of the principals in the assassination plot has been arrested.