Anything (such as a piece of furniture, or a software feature) that is built in, not added as an extra.

plural of built

simple past tense and past participle of build on

Alternative spelling of inbuilt

Built according to the specifications of a particular individual.

Made of sections or layers, one on top of the other. / Having increased in size, quantity, or intensity over time; accumulated. / Constructed or enhanced.

Alternative spelling of jerrybuilt

Made of a composite material. / (nautical, of a ship) Having wooden planks on a metal frame.

(computing) Any function that is provided as part of a high-level language and can be executed by a simple reference with specification of arguments

Of a building: built ad hoc, with whatever materials are to hand, and often just from ground level.