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n. pl. fo·cus·es or fo·ci (-sī′, -kī′) 1. a. The distinctness or clarity of an image rendered by an optical system. b. The state of maximum distinctness or clarity of such an image: in focus; out of focus. c. 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. 2. a. 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. b. See focal length. 3. a. A center of interest or activity: "Precisely how diet affects E. coli in livestock is the focus of current research" (Cindy Engel). b. Close or narrow attention; concentration: "He was forever taken aback by [New York's] pervasive atmosphere of purposefulness—the tight focus of its drivers, the brisk intensity of its pedestrians" (Anne Tyler). c. A condition in which something can be clearly apprehended or perceived: couldn't get the problem into focus. 4. Medicine The region of a localized bodily infection or disease. 5. Geology The point of origin of an earthquake. 6. Mathematics A fixed point whose relationship with a directrix determines a conic section. v. fo·cused, fo·cus·ing, fo·cus·es or fo·cussed or fo·cus·sing or fo·cus·ses v.tr. 1. To cause (light rays, for example) to converge on or toward a central point; concentrate. 2. a. 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. b. 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. v.intr. 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. [New Latin, from Latin, hearth (probably in reference to the fact that a lens or parabolic mirror can concentrate sunlight on a single point to start a fire).] focus·er n. |
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