(1):
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.
(2):
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.
(3):
A combining form prefix signifying first, primary, primordial; as, protomartyr, the first martyr; protomorphic, primitive in form; protoplast, a primordial organism; prototype, protozoan.