Source : Webster's Revised Unabridged Dictionary (1913)

Horopter \Ho*rop"ter\, n. [Gr. ? boundary + ? one who looks.]
   (Opt.)
   The line or surface in which are situated all the points
   which are seen single while the point of sight, or the
   adjustment of the eyes, remains unchanged.

         The sum of all the points which are seen single, while
         the point of sight remains unchanged, is called the
         horopter.                                --J. Le Conte.