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 for Principal focus
From The Collaborative International Dictionary of English v.0.48 :

  Focus \Fo"cus\ (f[=o]"k[u^]s), n.; pl. E. Focuses
     (f[=o]"k[u^]s*[e^]z), L. Foci (f[=o]"s[imac]). [L. focus
     hearth, fireplace; perh. akin to E. bake. Cf. Curfew,
     Fuel, Fusil the firearm.]
     1. (Opt.) A point in which the rays of light meet, after
        being reflected or refracted, and at which the image is
        formed; as, the focus of a lens or mirror.
        [1913 Webster]
     2. (Geom.) A point so related to a conic section and certain
        straight line called the directrix that the ratio of the
        distance between any point of the curve and the focus to
        the distance of the same point from the directrix is
        [1913 Webster]
     Note: Thus, in the ellipse FGHKLM, A is the focus and CD the
           directrix, when the ratios FA:FE, GA:GD, MA:MC, etc.,
           are all equal. So in the hyperbola, A is the focus and
           CD the directrix when the ratio HA:HK is constant for
           all points of the curve; and in the parabola, A is the
           focus and CD the directrix when the ratio BA:BC is
           constant. In the ellipse this ratio is less than unity,
           in the parabola equal to unity, and in the hyperbola
           greater than unity. The ellipse and hyperbola have each
           two foci, and two corresponding directrixes, and the
           parabola has one focus and one directrix. In the
           ellipse the sum of the two lines from any point of the
           curve to the two foci is constant; that is: AG + GB =
           AH + HB; and in the hyperbola the difference of the
           corresponding lines is constant. The diameter which
           passes through the foci of the ellipse is the major
           axis. The diameter which being produced passes through
           the foci of the hyperbola is the transverse axis. The
           middle point of the major or the transverse axis is the
           center of the curve. Certain other curves, as the
           lemniscate and the Cartesian ovals, have points called
           foci, possessing properties similar to those of the
           foci of conic sections. 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 a
           parabola, rays from the focus, after reflection at the
           curve, proceed in lines parallel to the axis. Thus rays
           from A in the ellipse are reflected to B; rays from A
           in the hyperbola are reflected toward L and M away from
           [1913 Webster]
     3. A central point; a point of concentration.
        [1913 Webster]
     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
     Focus tube (Phys.), a vacuum tube for R[oe]ntgen rays in
        which the cathode rays are focused upon the anticathode,
        for intensifying the effect.
     Principal focus, or Solar focus (Opt.), the focus for
        parallel rays.
        [1913 Webster]

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