Words with shared roots often end up with similar meanings in modern use. Principle and principal are two such words. Both of them entered English through Old French. Both have Latin roots—principium, which means “source” is the root of principle, and principalis, Latin for “first,” is the root of principal. If we were to dig a little bit deeper, we would see that the Latin roots can be traced to the same word—princeps, princip, which means “first”, or “chief.” Principle and principal are also pronounced the same way (PRIN-suh-pul), but that only adds to the confusion because the two words have different meanings:
Principle is a rule, a law, a guideline, or a fact;
Principal is the headmaster of a school, but also an adjective that means first or most important.
Principle and How to Use It
Principle is a word that’s always used as a noun, and it has a couple of meanings:
A fundamental truth upon which systems of beliefs and morals are formed:
I have been a selfish being all my life, in practice, though not in principle. As a child I was taught what was right, but I was not taught to correct my temper. I was given good principles, but left to follow them in pride and conceit.
—Jane Austen, Pride and Prejudice
A guiding rule that explains how something works:
When we asked Bill Nye the Science Guy if he thinks we are living in a computer-generated simulation, he turned to some basic scientific principles to justify his answer.
A scientific rule or law:
Water, following the principle of gravity, will run downhill thus forming new channels.
—Northern California News
Principal and How to Use It
Principal can be used both as an adjective and a noun. When used as an adjective, principal has two main meanings:
The first in order of importance:
Since it is the largest and probably the oldest of them, and since its contents are not found in any other manuscript, it can claim to be the foundation volume of English literature, one of the world’s principal cultural artefacts.
The originally invested amount of money:
Hit hard by the retrospective tax, Cairn Energy Plc of the UK has offered to pay 15 per cent of the Rs 10,247-crore principal amount in return for the government lifting its freeze on the 9.8 per cent shares it holds in its erstwhile subsidiary, Cairn India.
When used as a noun, principal also has a couple of meanings, including:
The head of an educational institution:
The principal of the troubled Boys and Girls High School in Brooklyn said on Thursday that he is leaving the post.
—The Wall Street Journal
The most important or highest ranked members of a troupe, a ballet company, or the headliners of an event:
Principals are the highest ranked dancers at a company and often play leading roles.
—The Japan Times
from Grammarly Blog