[foh-sahy, -kahy] /ˈfoʊ saɪ, -kaɪ/

a plural of .
[foh-kuh s] /ˈfoʊ kəs/
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.

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

plural of focus (n.).

1640s, from Latin focus “hearth, fireplace” (also, figuratively, “home, family”), of unknown origin, used in post-classical times for “fire” itself, taken by Kepler (1604) in a mathematical sense for “point of convergence,” perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer to “center of activity or energy” is first recorded 1796.

1775 in the literal sense; 1807 in the figurative sense, from focus (n.). Related: Focused; focusing; less commonly focussed; focussing.

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

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

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

[National Oceanic and Atmospheric Administration] Fisheries-Oceanography Coordinated Investigations
Fisheries Oceanography Cooperative Users System


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