Immutable means unable to be changed. When something is immutable, it stays exactly the same no matter what anyone does or how much time passes.

Some things in nature are often described as immutable: the speed of light in a vacuum never changes, and neither do the basic laws of physics as we understand them. Mathematical truths are immutable too. Two plus two always equals four, whether you're calculating it today or a thousand years from now.

You might hear someone say that a rule is immutable, meaning it cannot be bent or modified under any circumstances. A person's immutable characteristics are the ones they're born with and can't alter, like their DNA or the year they were born.

In computer programming, immutable data can't be modified after it's created. If you want to change it, you have to make an entirely new copy. This matters because it helps prevent accidental changes that could cause problems.

The opposite of immutable is mutable, which means changeable. Your opinions are mutable: they can shift as you learn new things. But some facts remain immutable no matter how we might feel about them.