A glissando is a musical technique in which the singer or player slides between notes. On a piano, this is accomplished by running the backside of one's hand up or down the keyboard. On string instruments, it is created by running a finger up or down the fingerboard while bowing a string. They are most difficult on wind instruments, and in fact are only possible on the clarinet, on which they are produced through a combination of fingering a chromatic scale while constantly modifying lip pressure. On trombones, they can be easily produced by moving the slide in or out.
The similar term "portamento" is used to distinguish a glissando which has no discernible pitches in between, explicitly excluding a glissando produced on a piano or a guitar.