n. pl. fo·cus·es or fo·ci (-sī′, -kī′)
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.
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.
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
1. To cause (light rays, for example) to converge on or toward a central point; concentrate.
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.
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).]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.