As a noun, principal refers to (1) one who holds a presiding position or rank, and (2) capital or property before interest, and it’s also an adjective meaning (3) first or most important in rank. The head of a primary or secondary school is a principal.
Principle is only a noun. In its primary sense, it refers to a basic truth, law, assumption, or rule.
Though the words sound alike and share a distant origin in the Latin princeps (meaning first or original), they come from separate French sources and have always been different words in English.1
The upcoming production of the webisode, “The Roomers” is now casting for principal actors. [My-Ishia’s Michigan Movie Industry Blog]
The school principal notified students’ parents about the case by e-mail and by posting a letter on the school Web site. [Houston Chronicle]
Although Rule 11 applies to only federal courts, the same principle applies to all plea hearings in all courts. [Pub Record]
But even if his principal objection is a matter of principle. [Tablet Mag]
It is a fundamental principle in the modern world that disabled people should be given the opportunity to participate in modern society with as few impediments as possible. [Emergency Planning]
1. Chambers Dictionary of Etymology, 1988. ↩