The Collaborative International Dictionary of English v.0.48:

Geometer \Ge*om"e*ter\, n. [F. g['e]om[`e]tre, L. geometres,
   geometra, fr. Gr. gewme`trhs, fr. ge`a, gh^, the earth +
   me`tron measure. See Meter measure.]
   1. One skilled in geometry; a geometrician; a mathematician.
      --I. Watts.
      [1913 Webster]

   2. (Zool.) Any species of geometrid moth; a geometrid.
      [1913 Webster]

WordNet (r) 3.0 (2006):

geometer
    n 1: a mathematician specializing in geometry [syn: geometer,
         geometrician]