ac·​tin | \ ˈak-tən \

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

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.

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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The first known use of actin was in 1942

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ac·​tin | \ ˈak-tən \

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

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excited commotion or publicity

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