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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.
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Also especially before a vowel, prot-.

Origin of proto-

<Greek, combining form representing prôtosfirst, superlative formed from pró;see pro-2
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023


What does proto- mean?

Proto– is a combining form used like a prefix meaning “first,” “foremost,” or “earliest form of.” In terms from chemistry, it specifically denotes the first in a series of compounds or the compound containing the minimum amount of an element. It is often used in scientific and technical terms, especially in biology.

Proto– comes from Greek prôtos, meaning “first.” The word proton, meaning “a positively charged elementary particle,” ultimately shares this same Greek root. The Latin cognate of prôtos is prīmus “first,” which is the source of primary and prime. Check out our entries for both words to learn more.

What are variants of proto-?

When combined with words or word elements that begin with a vowel, proto– becomes prot, as in protium. Want to know more? Read our Words That Use prot– article.

Examples of proto-

An example of a term from botany that features proto– is protoderm, “a thin outer layer of the meristem in embryos and growing points of roots and stems, which gives rise to the epidermis.”

The proto– part of the word means “earliest form of,” as we already know. The derm part of the word may also look familiar; it means “dermis,” which is the dense inner layer of skin. Protoderm literally translates to “earliest form of skin.”

What are some words that use the combining form proto-?

What are some other forms that proto– may be commonly confused with?

Break it down!

The form -type literally means, well, “type.” With this in mind, what does prototype literally mean?

How to use proto- in a sentence

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