Built-in means included as a permanent part of something from the very beginning, not added on later. A built-in bookshelf is constructed into the wall itself, not a separate piece of furniture you can move around. Built-in cabinets in a kitchen are attached and designed to fit the space perfectly.

Many things have built-in features that come standard. Your computer likely has a built-in camera and microphone. A car might have built-in navigation. These features are part of the original design, ready to use without buying or installing anything extra.

The term also describes natural abilities or characteristics. A cat has built-in hunting instincts: nobody teaches a kitten to stalk and pounce, it just knows how. Some people seem to have a built-in sense of direction and rarely get lost. When something is built-in, it's fundamental to how that thing works.

You might also hear about built-in advantages or disadvantages. Being tall gives basketball players a built-in advantage for reaching the hoop. Starting a race from the inside lane provides a built-in advantage because the distance is shorter. These advantages come automatically with the situation, not from anything you add or earn separately.