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The surface position directly beneath the center of a nuclear explosion.

hy′po·cen′tral (-sĕn′trəl) adj.


(ˈfoʊ kəs)

n., pl. -cus•es, -ci (-sī, -kī), n.
1. a central point, as of attention or activity.
2. a point at which rays of light, heat, or other radiation meet after being refracted or reflected.
a. the focal point of a lens.
b. the focal length of a lens.
c. the clear and sharply defined condition of an image.
d. the position of a viewed object or the adjustment of an optical device necessary to produce a clear image: out of focus.
4. (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. the point of origin of an earthquake.
6. the primary center from which a disease develops or in which it localizes.
7. to bring to a focus or into focus: to focus the lens of a camera.
8. to concentrate: to focus one's thoughts.
9. to become focused.
[1635–45; < Latin: fireplace, hearth]
fo′cus•a•ble, adj.