focus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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. n. See focal length.
3. n. The distinctness or clarity of an image rendered by an optical system.
4. n. The state of maximum distinctness or clarity of such an image: in focus; out of focus.
5. n. 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.
6. n. A center of interest or activity. See Synonyms at center.
7. n. 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).
8. n. A condition in which something can be clearly apprehended or perceived: couldn't get the problem into focus.
9. n. Pathology The region of a localized bodily infection or disease.
10. n. Geology The point of origin of an earthquake.
11. n. Mathematics A fixed point whose relationship with a directrix determines a conic section.
12. v. To cause (light rays, for example) to converge on or toward a central point; concentrate.
13. v. 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.
14. v. To adjust (a lens, for example) to produce a clear image.
15. v. To direct toward a particular point or purpose: focused all their attention on finding a solution to the problem.
16. v. To converge on or toward a central point of focus; be focused.
17. v. To adjust one's vision or an optical device so as to render a clear, distinct image.
18. v. To concentrate attention or energy: a campaign that focused on economic issues.

Century Dictionary and Cyclopedia

1. n. In optics, a point at which rays of light that originally diverged from one point meet again, or a point from which they appear to proceed. The former is called a real, the latter a virtual focus. The principal focus of a lens is the focus of rays striking the lens parallel to its axis. The conjugate foci of a mirror or lens are two points so situated that the rays emitted from a luminous body at either point are reflected (by the mirror) or refracted (by the lens) to the other. See conjugate mirror (under conjugate), lens, and mirror.
2. n. In geometry, a point from which the distances to any point of a given curve are in a syzygetic relation. Thus, the sum of the distances of any point of an ellipse from its foci is constant, and the difference of the distances of any point of a hyperbola from its foci is constant. A modern definition is that the foci are the intersections of common tangents of the curve and the absolute. In like manner, a focus of a surface is a point on the curve of intersection of common tangent planes of the surface and the absolute. See cuts under Cartesian and ellipse.
3. n. In the theory of perspective, with reference to two planes in perspective, one of four points—two, F₁ and F₂, on one plane, and two, f₁ and f₂, on the other—such that the angles between two points on the first plane measured at F₁ are equal to the angles between the corresponding points on the other plane measured at f₁, and so with the pair of foci F₂ and f₂. One pair of foci are called similar, because the angles are measured in the same direction on the two planes; the other pair are called dissimilar, because the angles are measured in opposite directions.
4. n. Figuratively (with a consciousness of the classical Latin meaning), a central or gathering point, like the fire or hearth of a household; the point at or about which anything is concentrated; a center of interest or attraction.
5. To bring or adjust to a focus; cause to be in focus; focalize; collect in one point; concentrate.
6. n. In pathology, a center of morbid action; one of the primary or principal lesions.

Wiktionary

1. n. countable, optics a point at which reflected or refracted rays of light converge.
2. n. countable, geometry a point of a conic at which rays reflected from a curve or surface converge.
3. n. uncountable, photography, cinematography The fact of the convergence of light on the photographic medium.
4. n. uncountable, photography, cinematography The quality of the convergence of light on the photographic medium.
5. n. uncountable concentration of attention.
6. n. countable, seismology the exact point of where an earthquake occurs, in three dimensions.
7. n. computing, graphical user interface The indicator of the currently active element in a user interface.
8. n. linguistics The most important word or phrase in a sentence or passage, or the one that imparts information
9. v. transitive To cause (rays of light, etc) to converge at a single point.
10. v. transitive To adjust (a lens, an optical instrument) in order to position an image with respect to the focal plane.
11. v. transitive To concentrate one's attention.
12. v. transitive To make (a liquid) less diluted.
13. v. intransitive To concentrate one’s attention.

GNU Webster's 1913

1. n. (Opt.) A point in which the rays of light meet, after being reflected or refracted, and at which the image is formed
2. n. (Geom.) A point so related to a conic section and certain straight line called the directrix that the ratio of the distance between any point of the curve and the focus to the distance of the same point from the directrix is constant.
3. n. A central point; a point of concentration.
4. v. To bring to a focus; to focalize; as, to focus a camera.

WordNet 3.0

1. n. a point of convergence of light (or other radiation) or a point from which it diverges
2. n. the concentration of attention or energy on something
3. n. maximum clarity or distinctness of an idea
4. n. maximum clarity or distinctness of an image rendered by an optical system
5. n. a central point or locus of an infection in an organism
6. v. direct one's attention on something
7. v. put (an image) into focus
8. n. a fixed reference point on the concave side of a conic section
9. v. become focussed or come into focus
10. v. cause to converge on or toward a central point
11. v. bring into focus or alignment; to converge or cause to converge; of ideas or emotions
12. n. special emphasis attached to something