Something which can be appended to something else.
A software extension or hardware peripheral that provides additional functions or customization for a core application or system.
Additional chips that can be purchased at specific times in tournament play.
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.