The curve thus traced will be an ellipse having the pins at the two points which are called its foci.
The villages of New England—the foci of blue laws and Puritanism.
The foci, mentioned above, were sharply demarcated from the surrounding liver tissue, which appeared to be intact.
The earth's orbit is an ellipse, one of the foci of which is occupied by the sun.
We have pointed out how the foci possess a geometrical significance which no other points enjoy.
Each one is called a focus, and an ellipse can only have one pair of foci.
Religion has two foci to determine,—the divine nature and the human.
They are, in fact, ellipsesone of the foci being the point about which the revolution is made.
Ages pass away, and these foci are the sun, the curve the orbit of planets.
At one of the points mentioned, which are the foci, let us fasten an orange.
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')
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.
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')