The Collaborative International Dictionary of English v.0.48:

Arioso \A`ri*o"so\, adv. & a. [It.] (Mus.)
   In the smooth and melodious style of an air; ariose.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arioso
    n 1: (music) a short recitative that is melodic but is not an
         aria