What’s the difference between principal and principle? The principle is of principal importance. Here’s the background for these close cousins, as well as related terms.
Principal derives by way of French from the Latin term principalis, meaning “first in importance.” In English, it initially referred to a ruler, but the word also came to be associated with an amount of money on which interest is paid, because that sum is first in terms of priority and the interest (one hopes) is a relative small amount.
Only about two hundred years ago did principal come to be associated with education; the principal, or first, teacher was often also head of the school, and “principal teacher” was simplified to principal. The word is still often used as an adjective, as in “principal violinist” or “principal consideration.”
Principle, by contrast, though it was originally merely a spelling variant, came to mean “proposition or truth,” and later “law of nature” and “rule of conduct.” And, unlike principal, it does not serve as an adjective except in the form of principled.
Prince and princess, and such derivatives as principality (princehood, or the country ruled by a prince), like principal and principle, ultimately stem from the Latin word princeps, meaning “first.” That’s why, although prince and princess usually refer to children of a monarch, prince itself is sometimes associated with someone primarily designated as a king (though no parallel relationship between queen and princess exists.)
Princeps itself comes from primus, from which English has developed the words prime, primer (pronounced with a long i when referring to an explosive cap and as PRIM-er when referring to a schoolbook), primary, and primate.
“Prima donna,” Italian for “first lady,” originally referred to the principal female singer in an opera; because of the association of such personages with outsized egos, the term was borrowed as a synonym for an arrogant, demanding person of either gender. (Its synonym, diva, is also Italian and means “goddess”; that word is related to divine.)
Premier and premiere are related to principal and principle as well; they started out as adjectives meaning “first.” “Premier minister,” an alternative to “prime minister,” was shortened to premier to refer to the chief executive of a nation, and “premiere presentation” was truncated to premiere to denote a first performance.
“The principal is your pal” is a venerable mnemonic that reminds us which spelling to use to refer to a person, but remember that, as mentioned above, principal can also refer to things such as funds.