Being an essential and permanent part of something.
Being an included feature that normally comes as an extra.
Constructed as a non-detachable part of a larger structure.
Anything (such as a piece of furniture, or a software feature) that is built in, not added as an extra.
Firmly established in thought or behaviour and difficult to change.
Having deep roots; (of a non-living object) deeply and firmly embedded (in the ground, etc.).