Built-in is an adjective that is often used to describe something that is an integral part of something else. Synonyms for built-in include inherent, integrated, incorporated, and intrinsic. These words all suggest that the feature or quality in question is an essential part of something else and cannot be easily separated or removed. For example, a car's air conditioning system is a built-in feature that is essential for comfort during hot weather. Similarly, a computer's operating system is an inherent part of its functionality, and without it, the computer would not work. Other synonyms for built-in include embedded, fixed, and innate, all of which suggest that the feature or quality in question is an essential, integral part of something else.