The Collaborative International Dictionary of English v.0.48:

Myosin \My"o*sin\, n. [Gr. my^s, myo`s a muscle.] (Physiol.
   Chem.)
   A protein present in muscle, serving as the principle
   contractile protein in muscle contraction.

   Note: It was earlier considered as being formed in the
         process of coagulation which takes place in rigor
         mortis. See Muscle plasma, under Plasma.
         [1913 Webster +PJC]

   Note: Myosin belongs to the group of globulins. It is
         insoluble in water, but soluble in dilute solution of
         salt, and is especially characterized by being
         completely precipitated by saturation of its solutions
         with salt.
         [1913 Webster]

WordNet (r) 3.0 (2006):

myosin
    n 1: the commonest protein in muscle; a globulin that combines
         with actin to form actomyosin