focus

focus is a Noun

Definition:

[1] A point in which the rays of light meet, after being reflected or refrcted, and at which the image is formed; as, the focus of a lens or mirror.

[2] A point so related to a conic section and certain straight line called the directrix that the ratio of the distace between any point of the curve and the focus to the distance of the same point from the directrix is constant.

[3] A central point; a point of concentration. Aplanatic focus. (Opt.) See under Aplanatic. -- Conjugate focus (Opt.), the focus for rays which have a sensible divergence, as from a near object; -- so called because the positions of the object and its image are interchangeable. -- Focus tube (Phys.), a vacuum tube for Roentgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect. -- Principal, or Solar, focus (Opt.), the focus for parallel rays.