That which is apposite; something suitable.
Strikingly appropriate or relevant; well suited to the circumstance or in relation to something.
Positioned at rest in respect to another, be it side-to-side, front-to-front, back-to-back, or even three-dimensionally: in apposition.
Related, homologous.
An item that one must own, or a feature that is essential to a service, etc.
Essential to own.