actin

1 of 2

noun

ac·​tin ˈak-tən 

: 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

Noun

Pilhofer and the postdoctoral researcher Florian Wollweber, working with Schleper and a team of collaborators, revealed the presence of a protein very much like eukaryotic actin in an Asgard, called Candidatus Lokiarchaeum ossiferum. —Veronique Greenwood, Quanta Magazine, 8 Sep. 2025

Plus: rich crazy people actin’ badly on a yacht, always a bankable sub-genre. —Kate Erbland, IndieWire, 26 Aug. 2025

In turn, the cytoskeleton relies on a type of actin protein, called filamentous or F-actin. —New Atlas, 2 Nov. 2024

In healthy muscle, small threadlike components called actin and myosin neatly line up next to each other. —Cindy Kuzma, SELF, 29 Oct. 2024

One color shows the cell nucleus (to mark where each cell is) and others show the structural component actin (which determines the shape of the cell) and the microtubules that are part of the communication and transport network within a cell. —Eva Amsen, Forbes, 17 Oct. 2024

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 *n̥k^wt-u-n-, a zero-grade derivative of *nok^wt-/*nek^wt- "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

Rhymes for actin

• prolactin

Kids Definition

actin

noun

ac·​tin ˈak-tən 

: a protein of muscle that with myosin is active in muscular contraction

Medical Definition

actin

noun

ac·​tin ˈak-tən 

: 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