The Collaborative International Dictionary of English v.0.48:

Colter \Col"ter\, n. [AS. culter, fr. L. culter plowshare,
   knife. Cf. Cutlass.]
   A knife or cutter, attached to the beam of a plow to cut the
   sward, in advance of the plowshare and moldboard. [Written
   also coulter.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

colter
    n 1: a sharp steel wedge that precedes the plow and cuts
         vertically through the soil [syn: colter, coulter]