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')