focus

focus [fō′kəs]

n.

pl. focuses or foci [fō′sī΄] [ModL, adopted in math. senses by KEPLER Johannes (1604) < L, fireplace, hearth < ? IE base * bhok-, to flame, burn > ? Arm boc̣, flame]

1. the point where rays of light, heat, etc. or waves of sound come together, or from which they spread or seem to spread; specif., the point where rays of light reflected by a mirror or refracted by a lens meet (called real focus) or the point where they would meet if prolonged backward through the lens or mirror (called virtual focus)

2. FOCAL LENGTH

3. an adjustment of the focal length to make a clear image [to bring a camera into focus]

4. any center of activity, attention, etc.

5. a part of the body where a disease process, as an infection, tumor, etc., is localized or most active

6. the starting point of an earthquake

7. Math.

a) either of the two fixed points used in determining an ellipse: see ELLIPSE

b) any analogous point for a parabola or hyperbola: see ECCENTRICITY, sense 3

vt.

focused or focussed, focusing or focussing

1. to bring into focus

2. to adjust the focal length of (the eye, a lens, etc.) in order to produce a clear image

3. to fix or settle on one thing; concentrate [to focus one's attention on a question]

vi.

1. to meet at a focus

2. to adjust one's eye or a lens so as to make a clear image

3. to direct one's thoughts or efforts; concentrate

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in focus

clear; distinct; sharply defined

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out of focus

indistinct; blurred

focuser

n.