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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-.
Origin of proto-
< Greek, combining form representing prôtos first, superlative formed from pró; see pro-2 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for proto-


combining form
indicating the first in time, order, or rank: protomartyr
primitive, ancestral, or original: prototype
indicating the reconstructed earliest stage of a language: Proto-Germanic
indicating the first in a series of chemical compounds: protoxide
indicating the parent of a chemical compound or an element: protactinium
Word Origin
from Greek prōtos first, from pro before; see pro-²
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for proto-

before vowels prot-, word-forming element meaning "first, source, parent, preceding, earliest form, original, basic," from Greek proto-, comb. form of protos "first," from PIE *pre-, from root *per- (1) "forward, through" (see per).

Online Etymology Dictionary, © 2010 Douglas Harper
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proto- in Medicine

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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