The Collaborative International Dictionary of English v.0.48:

Sostenuto \Sos`te*nu"to\, a. [It.] (Mus.)
   Sustained; -- applied to a movement or passage the sounds of
   which are to sustained to the utmost of the nominal value of
   the time; also, to a passage the tones of which are to be
   somewhat prolonged or protacted.
   [1913 Webster]