noun, plural fo·cus·es, fo·ci [foh-sahy, -kahy] /ˈfoʊ saɪ, -kaɪ/.

  1. a central point, as of attraction, attention, or activity: The need to prevent a nuclear war became the focus of all diplomatic efforts.
  2. Physics. a point at which rays of light, heat, or other radiation meet after being refracted or reflected.
  3. Optics.
    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.
  4. 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.
  5. Geology. the point of origin of an earthquake.
  6. Pathology. the primary center from which a disease develops or in which it localizes.

verb (used with object), fo·cused, fo·cus·ing or (especially British) fo·cussed, fo·cus·sing.

  1. to bring to a focus or into focus; cause to converge on a perceived point: to focus the lens of a camera.
  2. to concentrate: to focus one’s thoughts; to focus troop deployment in the east.

verb (used without object), fo·cused, fo·cus·ing or (especially British) fo·cussed, fo·cus·sing.

  1. to be or become focused: My eyes have trouble focusing on distant objects.
  2. to direct one’s attention or efforts: Students must focus in class.

noun plural -cuses or -ci (-saɪ, -kaɪ, -kiː)

  1. a point of convergence of light or other electromagnetic radiation, particles, sound waves, etc, or a point from which they appear to diverge
  2. another name for focal point (def. 1), focal length
  3. optics the state of an optical image when it is distinct and clearly defined or the state of an instrument producing this imagethe picture is in focus; the telescope is out of focus
  4. a point upon which attention, activity, etc, is directed or concentrated
  5. geometry a fixed reference point on the concave side of a conic section, used when defining its eccentricity
  6. the point beneath the earth’s surface at which an earthquake or underground nuclear explosion originatesCompare epicentre
  7. pathol the main site of an infection or a localized region of diseased tissue

verb -cuses, -cusing, -cused, -cusses, -cussing or -cussed

  1. to bring or come to a focus or into focus
  2. (tr often foll by on) to fix attention (on); concentrate

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.

n. pl. fo•cus•es

  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.focal point
  2. 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.


  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.

Plural focuses or foci (sī′, fōkī′)

  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.
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