ac·tin (?k?t?n)n. A protein that forms the microfilaments of the eukaryotic cytoskeleton and plays an important role in cell movement, shape, and internal organization. In muscle cells, it functions with myosin to produce contraction.[Latin ?ctus, motion; see act + -in.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.actin (?ækt?n) n (Biochemistry) a protein that participates in many kinds of cell movement, including muscle contraction, during which it interacts with filaments of a second protein, myosin[C20: from act + -in]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ac?tin (?æk t?n) n. a protein that functions in muscular contraction by combining with myosin. Compare actomyosin. [1940?45] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.