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pluralfo·ci\ˈfō-ˌsī also -ˌkī\alsofo·cus·es
Definition of FOCUS
a: a point at which rays (as of light, heat, or sound) converge or from which they diverge or appear to diverge; specifically: the point where the geometrical lines or their prolongations conforming to the rays diverging from or converging toward another point intersect and give rise to an image after reflection by a mirror or refraction by a lens or optical system
b: a point of convergence of a beam of particles (as electrons)