Caustic curve (Optics), a curve to which the ray of light, reflected or refracted by another curve, are tangents, the reflecting or refracting curve and the luminous point being in one plane. -- Caustic lime. See under Lime. -- Caustic potash, Caustic soda (Chem.), the solid hydroxides potash, KOH, and soda, NaOH, or solutions of the same. -- Caustic silver, nitrate of silver, lunar caustic. -- Caustic surface (Optics), a surface to which rays reflected or refracted by another surface are tangents. Caustic curves and surfaces are called catacaustic when formed by reflection, and diacaustic when formed by refraction.

Syn. -- Stinging; cutting; pungent; searching.