Definition of: focus

(fō′kəs) noun plural ·cus·es or ·ci (-sī)
1. Optics a The point to which a system of light rays converges after passage through a lens or other optical arrangement, or after reflection from a mirror, b The point from which such rays appear to diverge, c The place where a visual focus is clearly formed, as in the eye or a camera. The point of convergence of the rays is called the real focus; the point where diverging rays would meet if their directions were reversed is called the virtual focus.
2. Physics The meeting point of any system of rays, beams, or waves: an acoustic focus.
3. Geom. a One of two points, the sum or difference of whose distances to a conic section is a constant, b A point in some other curve, having similar properties.
4. Any central point: the focus of an earthquake.
—v.t. & v.i. fo·cused or fo·cussed, fo·cus·ing or fo·cus· sing
1. To adjust the focus of (the eye or an optical instrument).
2. To bring or come to a focus or point, as rays. [<L, hearth]
—fo′cal adjective
FOCI
A. Concavo–concave lens–showing light rays a, a, refracting as at b, b, and forming the virtual focus at c.
B. Convexo–convex lens–showing light rays d, d, converging to the principal focus at e.