1) Composed of myosin and actin 2) Protein 
(from the article `protein`) ...actin, the molecular weight of which is about 50,000; it forms 12 to 15 percent of the muscle proteins. Actin can exist in two forms—one, G-actin, ...
<chemical> A protein complex of actin and myosin occurring in muscle. It is the essential contractile substance of muscle. ... Chemical name: Actomyosins ... (12 Dec 1998) ... 
(ak″to-mi´o-sin) the complex of actin and myosin constituting muscle fibers and responsible for the contraction and relaxation of muscle.
Generally: a motor system that is thought to be based on actin and myosin. The essence of the motor system is that myosin makes transient contact with the actin filaments and undergoes a conformational change before releasing contact. The hydrolysis of ATP is coupled to movement, through the requirement for ATP to restore the configuration of myosin prior to repeating the cycle. More specifically: a viscous solution formed when actin and myosin s …
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Type: Term Pronunciation: ak′tō-mī′ō-sin Definitions: 1. A protein complex composed of actin and myosin; it is the essential contractile substance of muscle fiber, active with MgATP.
[n] - a protein complex in muscle fibers
noun a protein complex in muscle fibers; composed of myosin and actin; shortens when stimulated and causes muscle contractions
a complex protein, consisting of myosin and actin, that is the major constituent of skeletal muscle and is thought to interact with ATP to cause muscle contraction.