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proto-

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  1. 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.
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Also especially before a vowel, prot-.

Origin of proto-

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

proto-

sometimes before a vowel prot-

combining form
  1. indicating the first in time, order, or rankprotomartyr
  2. primitive, ancestral, or originalprototype
  3. indicating the reconstructed earliest stage of a languageProto-Germanic
  4. indicating the first in a series of chemical compoundsprotoxide
  5. indicating the parent of a chemical compound or an elementprotactinium
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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

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).

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

proto-

pref.
  1. First in time:prototype.
  2. First formed; primitive; original:protoplast.
  3. Having the least amount of a specified element or radical:protoporphyrin.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.