[ak-tuh n]

noun Biochemistry.

a globulin that is present in muscle plasma and that in connection with myosin plays an important role in muscle contraction.

Origin of actin

1940–45; perhaps act + -in2


variant of actino- before a vowel: actinism. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for actin

Historical Examples of actin

  • "The mare's actin' as if she'd a cup of tea, too," muttered her companion, Ned.


    W. A. Fraser

  • The nigher we got, the harder he stared; and the man in front was actin' similar in regards to him.

    The Depot Master

    Joseph C. Lincoln

  • And yet, if she wanted Jonadab, she was actin' mighty funny.

    The Depot Master

    Joseph C. Lincoln

  • All I should be able to think of was what she wore, or didn't wear, when she was actin' her parts.

    Kent Knowles: Quahaug

    Joseph C. Lincoln

  • Your Uncle Zoeth and me have been actin' as your pilots for a consider'ble spell.


    Joseph C. Lincoln

British Dictionary definitions for actin



a protein that participates in many kinds of cell movement, including muscle contraction, during which it interacts with filaments of a second protein, myosin

Word Origin for actin

C20: from act + -in
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

Medicine definitions for actin




One of the protein components found in muscle, existing as F-actin or G-actin, into which actomyosin can be split and which acts with myosin in muscle contraction.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Science definitions for actin



A protein found in all eukaryotic cells, forming filaments that make up a main component of the cell's supporting matrix or cytoskeleton. Actin and the protein myosin together make up the contractile units (called sarcomeres) of skeletal muscle fibers.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.