noun, plural fo·cus·es, fo·ci [foh-sahy, -kahy] /ˈfoʊ saɪ, -kaɪ/.
- 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.
verb (used with object), fo·cused, fo·cus·ing or (especially British) fo·cussed, fo·cus·sing.
verb (used without object), fo·cused, fo·cus·ing or (especially British) fo·cussed, fo·cus·sing.
Origin of focus
Synonyms for focus
Examples from the Web for foci
Historical Examples of foci
Instead, there are two focal points, called the foci, as you will see presently.Practical Mechanics for Boys
J. S. Zerbe
The villages of New England—the foci of blue laws and Puritanism.The Quadroon
The earth's orbit is an ellipse, one of the foci of which is occupied by the sun.Fragments of science, V. 1-2
Each one is called a focus, and an ellipse can only have one pair of foci.The Story of the Heavens
Robert Stawell Ball
It has two foci, S and S', fixed points, by the aid of which we may trace the curve.The Romance of Mathematics
noun plural -cuses or -ci (-saɪ, -kaɪ, -kiː)
verb -cuses, -cusing, -cused, -cusses, -cussing or -cussed
Word Origin for focus
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.