A Dream is a series of sensory imputs and emotions involuntarily occurring in the mind during REM sleep.
AI's will sometimes have the ability to dream added to their programs. Noonien Soong added the ability to dream to Data and The Doctor added a daydreaming subroutine to his program.
In 2367 the crew of the USS Enterprise (NCC-1701-D) went slowly insane as they were unable to dream while trapped in a Tyken's Rift. (TNG: "Night Terrors")
In 2374 the USS Voyager encountered the the Dream species, a race of aliens that lived out their lives in dreams. (VOY: "Waking Moments")
See also: