Dictionary.net

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"

A combining form prefix signifying first, primary, primordial; as, protomartyr, the first martyr; protomorphic, primitive in form; protoplast, a primordial organism; prototype, protozoan.

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.