The Collaborative International Dictionary of English v.0.48:

Euclidian \Eu*clid"i*an\, n.
   Related to Euclid, or to the geometry of Euclid.
   [1913 Webster]

   Euclidian space (Geom.), the kind of space to which the
      axioms and definitions of Euclid, relative to straight
      lines and parallel lines, apply; -- called also flat
      space, and homaloidal space.
      [1913 Webster]

WordNet (r) 3.0 (2006):

euclidian
    adj 1: relating to geometry as developed by Euclid; "Euclidian
           geometry" [syn: euclidian, euclidean]