ac·​tin | \ ˈak-tən How to pronounce actin (audio) \

Definition of actin

 (Entry 1 of 2)

: a cellular protein found especially in microfilaments (such as those comprising myofibrils) and active in muscular contraction, cellular movement, and maintenance of cell shape
variants: or actini- or actino-

Definition of actin- (Entry 2 of 2)

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: Noun

Wan believes the cell’s preference depends on the interplay between two elements of the cellular skeleton: actin and myosin. Tim Vernimmen, Quanta Magazine, "How Life Turns Asymmetric," 31 Jan. 2017 Nearly all multicellular animals, except for simple ones like sponges, depend on muscles to move, and all their muscles use some form of contracting fibers in which filaments of the proteins actin and myosin pull past one another to generate force. Quanta Magazine, "Too Small for Big Muscles, Tiny Animals Use Springs," 13 June 2018 Like a pasta maker, the scale cell squeezes out and assembles a lattice of actins. Gwen Pearson, WIRED, "How Butterflies Get Their Shine," 22 Dec. 2014

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'actin.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of actin


1942, in the meaning defined above

History and Etymology for actin


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).

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12 Jul 2019

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The first known use of actin was in 1942

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ac·​tin | \ ˈak-tən How to pronounce actin (audio) \

Medical Definition of actin

: 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 Encyclopedia article about actin

Comments on actin

What made you want to look up actin? Please tell us where you read or heard it (including the quote, if possible).


a usually brief trip or an expedition

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