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, especially before a vowel, prot-.

< Greek, combining form representing prôtos first, superlative formed from pró; see pro-2 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
proto- or (sometimes before a vowel) prot-
combining form
1.  indicating the first in time, order, or rank: protomartyr
2.  primitive, ancestral, or original: prototype
3.  indicating the reconstructed earliest stage of a language: Proto-Germanic
4.  indicating the first in a series of chemical compounds: protoxide
5.  indicating the parent of a chemical compound or an element: protactinium
[from Greek prōtos first, from pro before; see pro-²]
prot- or (sometimes before a vowel) prot-
combining form
[from Greek prōtos first, from pro before; see pro-²]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

comb. form meaning "first," from Gk. proto-, comb. form of protos "first," superlative of pro "before."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

proto- or prot-

  1. First in time: prototype.

  2. First formed; primitive; original: protoplast.

  3. Having the least amount of a specified element or radical: protoporphyrin.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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