Merriam-Webster Collegiate Dictionary - focus

 I. noun  (plural foci; also ~es)  Etymology: New Latin, from Latin, hearth  Date: 1644  1.  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)  2.  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 ~  d. direction 6c the team lost ~  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 ~ 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  • ~less adjective  II. verb  (~ed; also ~sed; ~ing; also ~sing)  Date: 1775  transitive verb  1.  a. to bring into ~  b. to adjust the ~ of (as the eye or a lens)  2. to cause to be concentrated ~ed their attention on the most urgent problems  3. to bring (as light rays) to a ~ ; concentrate  intransitive verb  1. to come to a ~ ; converge  2. to adjust one's eye or a camera to a particular range  3. to concentrate attention or effort  • ~able adjective  • ~er noun