: a subject that is being discussed or studied : the subject on which people's attention is focused
: a main purpose or interest
: a point at which rays of light, heat, or sound meet or from which they move apart or appear to move apart; especially : the point at which an image is formed by a mirror, a lens, etc.
pluralfo·ci\ˈfō-ˌsī also -ˌkī\alsofo·cus·es
Full 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)