(plural foci; also focuses)
Etymology: New Latin, from Latin, hearth
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)
a. focal length
b. adjustment for distinct vision; also the area that may be seen distinctly or resolved into a clear image
c. a state or condition permitting clear perception or understanding <tried to bring the issues into focus> d. direction 6c <the team lost focus> 3. one of the fixed points that with the corresponding directrix defines a conic section 4. a localized area of disease or the chief site of a generalized disease or infection 5. a. a center of activity, attraction, or attention <the focus of the meeting was drug abuse> b. a point of concentration 6. the place of origin of an earthquake or moonquake 7. directed attention ; emphasis • focusless adjective II. verb (focused; also focussed; focusing; also focussing) Date: 1775 transitive verb 1. a. to bring into focus b. to adjust the focus of (as the eye or a lens) 2. to cause to be concentrated <focused their attention on the most urgent problems> 3. to bring (as light rays) to a focus ; concentrate intransitive verb 1. to come to a focus ; converge 2. to adjust one's eye or a camera to a particular range 3. to concentrate attention or effort • focusable adjective • focuser noun
New Collegiate Dictionary. 2001.