principle

A principle is a fundamental truth, proposition, or value that serves as the foundation for a system of belief, behavior, or reasoning. Principles guide thinking and action and can function as the basis for laws, theories, or guidelines. The word derives from Latin principium, meaning "beginning" or "source," and from princeps, meaning "leading."

Principles can be normative, expressing how people ought to act (for example moral or ethical principles), or descriptive, capturing how things tend to behave under certain conditions (such as physical principles). They are often general and widely applicable, but they can also be specialized to domains such as science, engineering, or law. A principle is not the same as a rule, which prescribes a specific behavior in particular circumstances; a principle is broader and may underlie multiple rules.

Examples of widely cited principles include the principle of conservation of energy in physics, the principle of least action in mechanics, or the principle of fairness in ethics. In engineering and design, principles such as simplicity, reliability, and usability guide development. In mathematics, fundamental principles or axioms serve as starting points for proof, while in philosophy, first principles are basic propositions that cannot be deduced from more fundamental truths.

People sometimes confuse "principle" with "principal," which is typically used as an adjective meaning "main" or as a noun for a person in a position of authority. The plural form of principle is "principles."