fo·cus (fṓkəs)

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[Latin, hearth.]

noun: pl., -cus·es or -ci (-sī́, -kī́) 

1. 1. A point at which rays of light or other radiation converge or from which they appear to diverge, as after refraction or reflection in an optical system: the focus of a lens. Also called focal point
   2. See focal length
2. 1. The distinctness or clarity of an image rendered by an optical system.
   2. The state of maximum distinctness or clarity of such an image: in focus; out of focus.
   3. An apparatus used to adjust the focal length of an optical system in order to make an image distinct or clear: a camera with automatic focus.
3. A center of interest or activity. See synonyms at center
4. Close or narrow attention; concentration: “He was forever taken aback by pervasive atmosphere of purposefulness—the tight focus of its drivers, the brisk intensity of its pedestrians” (Anne Tyler)
5. A condition in which something can be clearly apprehended or perceived: couldn't get the problem into focus.
6. Pathology The region of a localized bodily infection or disease.
7. Geology The point of origin of an earthquake.
8. Mathematics A fixed point whose relationship with a directrix determines a conic section.

verb: -cused or -cussed, -cus·ing or -cus·sing, -cus·es or -cus·ses 

transitive verb 

1. To cause (light rays, for example) to converge on or toward a central point; concentrate.
2. 1. To render (an object or image) in clear outline or sharp detail by adjustment of one's vision or an optical device; bring into focus.
   2. To adjust (a lens, for example) to produce a clear image.
3. To direct toward a particular point or purpose: focused all their attention on finding a solution to the problem.

intransitive verb 

1. To converge on or toward a central point of focus; be focused.
2. To adjust one's vision or an optical device so as to render a clear, distinct image.
3. To concentrate attention or energy: a campaign that focused on economic issues.

derivatives

fócus·er

noun

noun 

The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus