a combining form meaning "first," "foremost," "earliest form of," used in the formation of compound words ( protomartyr; protolithic; protoplasm ), specialized in chemical terminology to denote the first of a series of compounds, or the one containing the minimum amount of an element.
Also, esp. before a vowel, prot- .
[ prôtos FIRST, superl. formed from pró; see PRO 2 - ]