actin

1 of 2

noun

ac·​tin ˈak-tən How to pronounce actin (audio)
: a cellular protein found especially in microfilaments (such as those comprising myofibrils) and active in muscular contraction, cellular movement, and maintenance of cell shape

actin-

2 of 2

combining form

variants or actini- or actino-
1
: having a radiate form
actinolite
2
: actinic radiation (such as X-rays)
actinometer

Examples of actin in a Sentence

Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Noun
In healthy muscle, small threadlike components called actin and myosin neatly line up next to each other. — Cindy Kuzma, SELF, 29 Oct. 2024 The head contains the ATP-binding site, a small converter region, and the actin-binding site. — Encyclopedia Britannica, 3 Apr. 2026 These bulges are made from actin, a protein that helps shape the cell, and they weren’t stopped by certain inhibitors, meaning the process may be similar to other cell activities. — Pranjal Malewar, New Atlas, 12 Dec. 2025 Wada had begun experimenting with chloroplast motion back in the 1990s by looking into how the organelles move or anchor themselves by sprouting cytoskeletal fibers, including actin and microtubules. — Quanta Magazine, 4 May 2026 See All Example Sentences for actin

Word History

Etymology

Noun

probably from act entry 2 or act(ivate) + -in entry 1

Combining form

borrowed from Greek aktino-, from aktīn-, aktís "ray, beam, spoke of a wheel"

Note: The Greek word has traditionally been linked with Sanskrit aktú-, conjectured to mean "twilight," and with Germanic *uŋhtwōn- "last part of the night, dawn," whence Old English ūhte "twilight, daybreak," Old High German uohta "dawn," Old Norse ōtta "last part of the night," Gothic uhtwo "dawn," from Indo-European *n̥kwt-u-n-, a zero-grade derivative of *nokwt-/*nekwt- "night" (see night entry 1). Robert Beekes (Etymological Dictionary of Greek, Leiden, 2009) rejects this connection on the grounds that aktís "shows no trace of a labiovelar." In reference to actinic radiation, the formative actino- was probably first used by John herschel in actinometer (1833).

First Known Use

Noun

1942, in the meaning defined above

Time Traveler
The first known use of actin was in 1942

Rhymes for actin

See All Rhymes for actin

Browse Nearby Words

actify
actin
actin-
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Cite this Entry

“Actin.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/actin. Accessed 7 Jul. 2026.

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Kids Definition

actin

noun
ac·​tin ˈak-tən How to pronounce actin (audio)
: a protein of muscle that with myosin is active in muscular contraction

Medical Definition

actin

noun
ac·​tin ˈak-tən How to pronounce actin (audio)
: a protein found especially in microfilaments (as those comprising myofibrils) and active in muscular contraction, cellular movement, and maintenance of cell shape see f-actin, g-actin

More from Merriam-Webster on actin

Nglish: Translation of actin for Spanish Speakers

Britannica.com: Encyclopedia article about actin

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