The Collaborative International Dictionary of English v.0.48:

Portamento \Por`ta*men"to\, n. [It., fr. portare to carry.]
   (Mus.)
   In singing, or in the use of the bow, a gradual carrying or
   lifting of the voice or sound very smoothly from one note to
   another; a gliding from tone to tone.
   [1913 Webster]