The Collaborative International Dictionary of English v.0.48:

Proto- \Pro"to-\ [Gr. prw^tos first, a superl. fr. pro` before.
   See Pro-.]
   1. A combining form prefix signifying first, primary,
      primordial; as, protomartyr, the first martyr;
      protomorphic, primitive in form; protoplast, a primordial
      organism; prototype, protozoan.
      [1913 Webster]

   2. (Chem.)
      (a) Denoting the first or lowest of a series, or the one
          having the smallest amount of the element to the name
          of which it is prefixed; as protoxide, protochloride,
          etc.
      (b) Sometimes used as equivalent to mono-, as indicating
          that the compound has but one atom of the element to
          the name of which it is prefixed. Also used
          adjectively.
          [1913 Webster]

WordNet (r) 3.0 (2006):

proto
    adj 1: indicating the first or earliest or original; "`proto' is
           a combining form in a word like `protolanguage' that
           refers to the hypothetical ancestor of another language
           or group of languages"