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 Unabridged Dictionary, © Random House, Inc. 2020

British Dictionary definitions for proto-


sometimes before a vowel prot-

combining form

indicating the first in time, order, or rankprotomartyr
primitive, ancestral, or originalprototype
indicating the reconstructed earliest stage of a languageProto-Germanic
indicating the first in a series of chemical compoundsprotoxide
indicating the parent of a chemical compound or an elementprotactinium

Word Origin for proto-

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

Medical definitions for proto-



First in time:prototype.
First formed; primitive; original:protoplast.
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.