focus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The distinctness or clarity of an image rendered by an optical system.
• noun The state of maximum distinctness or clarity of such an image.
• noun An apparatus used to adjust the focal length of an optical system in order to make an image distinct or clear.
• noun 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.
• noun A center of interest or activity.
• noun Close or narrow attention; concentration.
• noun A condition in which something can be clearly apprehended or perceived.
• noun Medicine The region of a localized bodily infection or disease.
• noun Geology The point of origin of an earthquake.
• noun Mathematics A fixed point whose relationship with a directrix determines a conic section.
• intransitive verb To cause (light rays, for example) to converge on or toward a central point; concentrate.
• intransitive verb 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.
• intransitive verb To adjust (a lens, for example) to produce a clear image.
• intransitive verb To direct toward a particular point or purpose.
• intransitive verb To converge on or toward a central point of focus; be focused.
• intransitive verb To adjust one's vision or an optical device so as to render a clear, distinct image.
• intransitive verb To concentrate attention or energy.

from The Century Dictionary.

• To bring or adjust to a focus; cause to be in focus; focalize; collect in one point; concentrate.
• noun 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.
• noun In geometry, a point from which the distances to any point of a given curve are in a syzygetic relation.
• noun 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.
• noun 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.
• noun In pathology, a center of morbid action; one of the primary or principal lesions.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Opt.) A point in which the rays of light meet, after being reflected or refracted, and at which the image is formed
• noun (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.
• noun A central point; a point of concentration.
• noun (Opt.) See under Aplanatic.
• noun (Opt.) the focus for rays which have a sensible divergence, as from a near object; -- so called because the positions of the object and its image are interchangeable.
• noun (Phys.) a vacuum tube for Rœntgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect.
• noun (Opt.) the focus for parallel rays.
• transitive verb To bring to a focus; to focalize; as, to focus a camera.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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