Glissando is a musical term that refers to sliding between two pitches while filling in all the intermediate pitches rapidly. On the piano, for example, all notes between the starting note and the ending note would be played by dragging the knuckle or fingernail across the piano keyboard, striking all the keys. On instruments that can produce any pitch, such as the violin, the flute, or the tuba, a glissando will technically include all possible pitches in between the starting and the ending pitch.
Any instrument that can produce a variegated pitch can be used to play a glissando. Pianists play them by running their knuckles or nails rapidly over a succession of notes. String instrumentalists play them by sliding their finger along the string from one note to another. The musical saw is played entirely with glissando, as all transitions between notes include the notes in between. Glissando can even be performed on drums--for example, sliding ones fingers across the the drumskin of a conga drum produces a tone that moves around in pitch known as a moose call.
In sheet music, glissandi are represented by a line beginning at the starting pitch of the glissando and ending at the final pitch. The line may be straight or wavy. The notation for the glissando may also appear as the text "gliss." below the starting and ending pitches.
One of the most well-known examples of glissando is the clarinet opening of George Gershwin's Rhapsody in Blue. After a short trill and an ascending scale, the clarinet slides up to the first note of the introductory melody using a glissando. In the 4th section of Ravel's Rapsodie Espagnole for Orchestra, Feria, trombone glissandi punctuate the last few bars. Bartok's Concerto for Orchestra includes a glissando for the bass trombone that requires a double valve bass trombone to be played in place of the standard single valve double bass trombone.
In his collection Ten Etudes for piano, Virgil Thomson included an exercise called the Double Glissando Etude, which contains many examples of glissando and is designed to help pianists gain proficiency with the technique. Mozart's La Tartine de Beurre for solo piano contains many glissandi in its main theme. In Gyorgy Ligeti's Ramifications for 12 solo strings, glissandi are used throughout the duration of the piece to create an otherworldly atmosphere.