Source : Webster's Revised Unabridged Dictionary (1913)

Glissando \Glis*san"do\, n. & a. [As if It. = Fr. glissant
   sliding.] (Mus.)
   A gliding effect; gliding.

Source : WordNet®

glissando
     n : a rapid series of ascending or descending notes on the
         musical scale
     adv : (musical direction) in the manner of a glissando (with a
           rapidly executed series of notes); "this should be
           played glissando, please"
     [also: {glissandi} (pl)]