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.
Incapable of being canceled, lost, or forgotten.
Having the quality of being difficult to delete, remove, wash away, blot out, or efface.
Incapable of being annulled.