The principle definition is a basic truth, rule or belief that other ideas or actions are built on. If you refuse to copy a friend's homework even though no one would find out, you are acting on a principle: honesty matters to you whether or not you are watched.
The word works in two worlds. In science and engineering, a principle is a general law about how things work, such as the principle that hot air rises. In ethics and daily life, it is a personal rule of conduct. Both senses point to the same idea: a foundation that stays steady while details change.
Entry at a glance
Meaning
A basic truth, law or moral rule that guides action or explains how something worksPart of speech
Noun (countable: principles)Pronunciation
PRIN-suh-puhl, the same as principalWord forms
principled (adjective), unprincipled (opposite)Origin
Latin principium, beginning, through Old French; in English from the late 1300sKey phrases
in principle, on principle, a matter of principle, first principlesDo not confuse
principal (main; head of a school; a sum of money)
Principle definition: four senses, from morals to machines
- A moral rule or belief. "She has strong principles about wasting food." Here principles are personal standards of right and wrong, close to ethics and integrity.
- A basic law of nature or science. "Airplanes fly because of principles of aerodynamics."
- A basic idea behind a system or method. "The principle of a democracy puts ultimate authority in ordinary citizens." "The design follows the principle of keeping things simple."
- A starting point or source (older and technical). In chemistry, an "active principle" is the ingredient in a plant or drug that produces its effect.
A short definition of principle, if you need to define principle in one sentence: it is the deep rule that sits underneath behaviour, beliefs or machines.
Principle meaning in common phrases
| Phrase | Meaning | Example |
|---|---|---|
| In principle | In theory, or agreed in general but details not settled | The plan is approved in principle. |
| On principle | Because of a belief, not for practical reasons | He never borrows money on principle. |
| A matter of principle | Something you do because it is right, even at a cost | Returning the extra change was a matter of principle. |
| First principles | The most basic facts from which you reason | Let's work out the cost from first principles. |
| A person of principle | Someone with firm moral standards | Our coach is a woman of principle. |
Principle vs principal
These two sound exactly alike, which causes endless mistakes. Use this check:
- Principle is always a noun meaning a rule, law or belief. Both end in -le: principle, rule.
- Principal can be an adjective meaning main (the principal reason), or a noun for the head of a school, a main person in a deal, or the original sum of a loan.
A memory trick many teachers use: the principal is your pal, the person in charge of the school. So "the school principal" and "the principle of fairness" are both correct, but never "a man of principal".
From Latin principium
English has had principle since the 1300s. It arrived via Old French principe, from Latin principium, a beginning or foundation. Principium in turn comes from princeps, the first or chief, the same root as prince. The earliest English sense, origin or source, has faded. The sense of a rule of conduct grew strong by the 1500s, and the scientific sense of a general law of nature became common later. The -le ending probably developed by analogy with words like participle.
Principle in sentences
- The basic principle of saving is simple: spend less than you earn.
- He quit the job because it went against his principles.
- The engineer explained the principle behind the new water filter using a coffee strainer.
- In principle, anyone can apply, but the form is only in English.
- She is principled enough to admit when she is wrong.
Close words and opposites
Related words include value (something you care about), standard (a level you expect), rule (a stated instruction), law (in science, a tested general statement) and conviction (a firmly held belief). A dogma is a principle that a group insists cannot be questioned.
There is no single opposite noun, but the adjective unprincipled describes someone with no moral standards. Many other common words get the same treatment in our core vocabulary hub.
Frequently asked questions
What does principle mean in simple words?
A principle is a basic rule or belief. It can be a moral rule you live by, or a basic fact about how something in nature or a machine works.
Is it principle or principal?
Use principle for a rule or belief. Use principal for the head of a school, the main thing or the original amount of a loan. They sound the same but are different words.
What does in principle mean?
It means in theory or in general. If a manager agrees in principle, they like the idea but have not approved every detail yet.
What is a person of principle?
Someone who sticks to clear moral standards, even when that is hard or costly. It is a compliment.
What does principled mean?
Principled is the adjective for someone acting on firm values: a principled leader, a principled decision. Its opposite is unprincipled.
How is principle pronounced?
PRIN-suh-puhl, putting the weight on PRIN. Because principal is said identically, listeners rely on context to know which word you mean.