noun, plural focuses, foci
[foh-sahy, -kahy] /ˈfoʊ saɪ, -kaɪ/ (Show IPA)
a central point, as of attraction, attention, or activity:
The need to prevent a nuclear war became the focus of all diplomatic efforts.
Physics. a point at which rays of light, heat, or other radiation meet after being refracted or reflected.

the focal point of a lens, on which rays converge or from which they deviate.
the focal length of a lens; the distance from a focal point to a corresponding principal plane.
the clear and sharply defined condition of an image.
the position of a viewed object or the adjustment of an optical device necessary to produce a clear image:
in focus; out of focus.

Geometry. (of a conic section) a point having the property that the distances from any point on a curve to it and to a fixed line have a constant ratio for all points on the curve.
Geology. the point of origin of an earthquake.
Pathology. the primary center from which a disease develops or in which it localizes.
verb (used with object), focused, focusing or (especially British) focussed, focussing.
to bring to a focus or into focus; cause to converge on a perceived point:
to focus the lens of a camera.
to concentrate: to focus one’s thoughts;
to focus troop deployment in the east.
verb (used without object), focused, focusing or (especially British) focussed, focussing.
to be or become focused:
My eyes have trouble focusing on distant objects.
to direct one’s attention or efforts:
Students must focus in class.
noun (pl) -cuses, -ci (-saɪ; -kaɪ; -kiː)
a point of convergence of light or other electromagnetic radiation, particles, sound waves, etc, or a point from which they appear to diverge
another name for focal point (sense 1), focal length
(optics) the state of an optical image when it is distinct and clearly defined or the state of an instrument producing this image: the picture is in focus, the telescope is out of focus
a point upon which attention, activity, etc, is directed or concentrated
(geometry) a fixed reference point on the concave side of a conic section, used when defining its eccentricity
the point beneath the earth’s surface at which an earthquake or underground nuclear explosion originates Compare epicentre
(pathol) the main site of an infection or a localized region of diseased tissue
verb -cuses, -cusing, -cused, -cusses, -cussing, -cussed
to bring or come to a focus or into focus
(transitive) often foll by on. to fix attention (on); concentrate

focus fo·cus (fō’kəs)
n. pl. fo·cus·es or fo·ci (-sī’, -kī’)

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. Also called focal point.

See focal length.

The distinctness or clarity of an image rendered by an optical system.

The state of maximum distinctness or clarity of such an image.

An apparatus used to adjust the focal length of an optical system in order to make an image distinct or clear.

The region of a localized bodily infection or disease.

v. fo·cused or fo·cussed, fo·cus·ing or fo·cus·sing, fo·cus·es or fo·cus·ses

To cause light rays or other radiation to converge on or toward a central point; concentrate.

To render an object or image in clear outline or sharp detail by adjustment of one’s vision or an optical device.

To adjust a lens or instrument to produce a clear image.

To converge on or toward a central point of focus; be focused.

Plural focuses or foci (fō’sī’, fō’kī’)

The degree of clarity with which an eye or optical instrument produces an image.

See focal point.

A central point or region, such as the point at which an earthquake starts.

Mathematics A fixed point or one of a pair of fixed points used in generating a curve such as an ellipse, parabola, or hyperbola.

The region of a localized bodily infection or disease.

Fisheries Oceanography Cooperative Users System


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