The Collaborative International Dictionary of English v.0.48:

Margarine \Mar"ga*rine\ (m[aum]r"j[u^]*r[i^]n;
   m[aum]r`j[u^]*r[=e]n"), n. [F.; see margarin.]
   1. A processed food product used as an inexpensive substitute
      for butter, made primarily from refined vegetable oils,
      sometimes including animal fats, and churned with skim
      milk to form a semisolid emulsion; also called
      oleomargarine; artificial butter.
      [Webster 1913 Suppl. +PJC]

            The word margarine shall mean all substances,
            whether compounds or otherwise, prepared in
            imitation of butter, and whether mixed with butter
            or not.                               --Margarine
                                                  Act, 1887 (50
                                                  & 51 Vict. c.
                                                  29).
      [Webster 1913 Suppl.]

   2. Margarin.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

margarine
    n 1: a spread made chiefly from vegetable oils and used as a
         substitute for butter [syn: margarine, margarin,
         oleo, oleomargarine, marge]