A principle (n.) is a fundamental idea, belief, philosophy, or rule:
The word principal has three meanings:
1. As an adjective: the main or most important The lake is the principal water source for the city.
2. As a noun: the director of an elementary, middle, or high school Students who misbehave are sent to the principal’s office.
3. As a noun (finance): the original amount of a debt (not counting the interest
– the extra money that accumulates over time as the debt is not paid):
At a 10% interest rate, a principal of $1,000 will accumulate $100 of interest.