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.
Being very important, or key to something.
Having or containing the centre of something.
Being in the centre.
Exerting its action towards the peripheral organs.
centre