Fō'cus1, n.
n.; Eng. pl. FŌ'CUS-ES; Lat. pl. FŌ'ÇĪ. [Lat. focus, hearth, fire-place; whence It. fuoco, Sp. fuego, Pg. fogo, Pr. fuec, Fr. feu, fire.]
Sense 1. (Opt.) A point in which the rays of light meet, after being reflected or refracted; as, the focus of a lens or mirror.
Sense 2. (Geom.) A point on the principal axis of a conic section, such that the double ordinate to the axis through the point shall be equal to the parameter of the curve.
☞ In an ellipse, rays of light coming from one focus, and reflected from the curve, proceed in lines directed toward the other; in an hyperbola, in lines directed from the other; in the parabola, rays from the focus, after reflection at the curve, proceed in lines parallel to the axis.
Sense 3. A central point; a point of concentration.
Principal or solar focus (Opt.), the focus for parallel rays.