[1] : The Collaborative International Dictionary of English v.0.48

focus \fo"cus\ f=o"ku^s, n.; pl. e. focuses
   f=o"ku^se^z, l. foci f=o"simac. 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
      constant.
      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
         b.
         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
      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 focus, or solar focus opt., the focus for
      parallel rays.
      1913 webster
see also:
focuses foci curfew fuel fusil aplanatic focus 
aplanatic conjugate focus focus tube principal focus solar focus 

[2] : The Collaborative International Dictionary of English v.0.48

conjugate \con"jugate\, a. l. conjugatus, p. p. or conjugare
   to unite; con- + jugare to join, yoke, marry, jugum yoke;
   akin to jungere to join. see join.
   1. united in pairs; yoked together; coupled.
      1913 webster

   2. bot. in single pairs; coupled.
      1913 webster

   3. chem. containing two or more compounds or radicals
      supposed to act the part of a single one. r.
      1913 webster

   4. gram. agreeing in derivation and radical signification;
      -- said of words.
      1913 webster

   5. math. presenting themselves simultaneously and having
      reciprocal properties; -- frequently used in pure and
      applied mathematics with reference to two quantities,
      points, lines, axes, curves, etc.
      1913 webster

   conjugate axis of a hyperbola math., the line through the
      center of the curve, perpendicular to the line through the
      two foci.

   conjugate diameters conic sections, two diameters of an
      ellipse or hyperbola such that each bisects all chords
      drawn parallel to the other.

   conjugate focus opt. see under focus.

   conjugate mirrors optics, two mirrors so placed that rays
      from the focus of one are received at the focus of the
      other, especially two concave mirrors so placed that rays
      proceeding from the principal focus of one and reflected
      in a parallel beam are received upon the other and brought
      to the principal focus.

   conjugate point geom., an acnode. see acnode, and
      double point.

   self-conjugate triangle conic sections, a triangle each
      of whose vertices is the pole of the opposite side with
      reference to a conic.
      1913 webster
see also:
join conjugate axis of a hyperbola conjugate diameters conjugate focus focus conjugate mirrors 
conjugate point acnode double point self-conjugate triangle 

Conjugate >> C Words
Conjugate, definition of term: Conjugate
conjugate_pag1.html

Focus >> F Words
Focus, definition of term: Focus
focus_pag1.html

Focus On >> F Words
Focus On, definition of term: Focus On
focus+on_pag1.html

Virtual Focus >> V Words
Virtual Focus, definition of term: Virtual Focus
virtual+focus_pag1.html