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 operative person or thing.
Any of a class of behaviors that produce consequences by operating (i.e., acting) upon the environment.
That operates to produce an effect.