A principle is a fundamental truth or rule that guides how something works or how someone should act. When scientists discover a principle of physics, like gravity, they've found a rule that explains how the universe operates. When someone acts on principle, they're following their deep beliefs about right and wrong, even when it's difficult.

Principles are different from ordinary rules. A school rule might say “no running in the hallways,” but the principle behind it is keeping everyone safe. Understanding the principle helps you know why the rule exists. In math, when you learn the principles of multiplication, you're learning the fundamental ideas that make all multiplication problems work the same way.

People often talk about having strong principles, meaning they have clear beliefs they won't abandon. A person of principle sticks to what they believe is right even under pressure. If your friend refuses to copy homework because of their principles about honesty, they're putting their values above convenience.

The phrase “in principle” means something should work according to the basic rules or logic, even if you haven't tried it yet. Your plan to build a treehouse might work in principle, but you'll still need to test whether the branches can hold the weight.

Principles shape everything from how we treat others to how machines function. They're the foundation that everything else is built on.