The Collaborative International Dictionary of English v.0.48:

Diacaustic \Di`a*caus"tic\, a. [Pref. dia- + caustic.] (Opt.)
   Pertaining to, or possessing the properties of, a species of
   caustic curves formed by refraction. See Caustic surface,
   under Caustic.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diacaustic \Di`a*caus"tic\, n.
   1. (Med.) That which burns by refraction, as a double convex
      lens, or the sun's rays concentrated by such a lens,
      sometimes used as a cautery.
      [1913 Webster]

   2. (Math.) A curved formed by the consecutive intersections
      of rays of light refracted through a lens. Diachylon