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[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.
  1. the focal point of a lens, on which rays converge or from which they deviate.
  2. the focal length of a lens; the distance from a focal point to a corresponding principal plane.
  3. the clear and sharply defined condition of an image.
  4. 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.
Origin of focus
1635-45; < Latin: fireplace, hearth
Related forms
focusable, adjective
focuser, noun
misfocus, verb, misfocused, misfocusing or (especially British) misfocussed, misfocussing.
misfocused, adjective
misfocussed, adjective
overfocus, verb (used with object), overfocused, overfocusing or (especially British) overfocussed, overfocussing.
refocus, verb, refocused, refocusing or (especially British) refocussed, refocussing.
self-focused, adjective
self-focusing, adjective
self-focussed, adjective
self-focussing, adjective
unfocusing, adjective
unfocussing, adjective
well-focused, adjective
well-focussed, adjective
1. center, heart, core, nucleus. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for focussing
Contemporary Examples
Historical Examples
  • “I am looking,” replied the boy, who was focussing the objects that had startled his cousin.

    Dead Man's Land George Manville Fenn
  • Only, you are focussing it from one end, and I from another.

    The Making of a Prig Evelyn Sharp
  • The focussing screen should be ruled as accompanying diagram.

  • You talk of 'focussing,' but what else, even in those minutes, were you in fact engaged in?

    The Sacred Fount Henry James
  • They are equally useful in focussing the shadows, or in giving them variety.

  • The binocular was handed at once, and after a great deal of focussing, handed back.

    Stan Lynn George Manville Fenn
  • The greatest care should be taken in focussing the telescope.

    Half-hours with the Telescope Richard A. Proctor
  • What provision is made for moving or focussing the lamp in the reflector?

  • At the orbital velocity of the ship, focussing on the star was indeed a difficult thing to do.

    Islands of Space John W Campbell
British Dictionary definitions for focussing


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
Derived Forms
focusable, adjective
focuser, noun
Word Origin
C17: via New Latin from Latin: hearth, fireplace
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for focussing



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.

Online Etymology Dictionary, © 2010 Douglas Harper
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focussing in Medicine

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

  1. 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.

  2. See focal length.

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

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

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

  6. 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
  1. To cause light rays or other radiation to converge on or toward a central point; concentrate.

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

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

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

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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focussing in Science
Plural focuses or foci (fō'sī', fō'kī')
  1. The degree of clarity with which an eye or optical instrument produces an image.

  2. See focal point.

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

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

  5. The region of a localized bodily infection or disease.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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