The Collaborative International Dictionary of English v.0.48:
Proto- \Pro"to-\ [Gr. prw^tos first, a superl. fr. pro` before.
1. A combining form prefix signifying first, primary,
primordial; as, protomartyr, the first martyr;
protomorphic, primitive in form; protoplast, a primordial
organism; prototype, protozoan.
(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,
(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