"Dream", sometimes referred to as "Dream (When You're Feeling Blue)", is a jazz and pop standard with words and music written by Johnny Mercer in 1944. It has been and performed by many artists, with the most popular versions of this song recorded by The Pied Pipers, Frank Sinatra, and Roy Orbison.

For Capitol Records, The Pied Pipers, with lead singer June Hutton, made a version of "Dream" (catalog number 185, with the flip side "Tabby the Cat") which became a major hit in 1945. Vocal group The Skylarks, singing with Ray Anthony's orchestra, would revive this ballad in the 1955 Fred Astaire-Leslie Caron musical film, Daddy Long Legs.

In 1970, a vocal quartet which included lead singer Sue Allen (who sang with The Pied Pipers in the 1950s), recorded it with the same arrangement as the 1945 hit version, for Time-Life Records. On October 22, 2008, this version was used in the teaser trailer for the 2K Games Bioshock sequel, BioShock 2: Sea of Dreams.

"Dream" was also recorded (on April 14, 1958) by Betty Johnson (issued by Atlantic Records as catalog number 1186, with the flip side "How Much") in a version that spent seven weeks on the charts: #19 on the Billboard chart of songs most played by disc jockeys and #58 on the Billboard top 100 chart. Roy Orbison included a cover of the song on his popular and critically acclaimed 1963 album for Monument Records, In Dreams. More recently, Orbison's version was resurrected for the soundtrack to the 1998 film, You've Got Mail.

Dreams are successions of images, ideas, emotions, and sensations that occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not definitively understood, though they have been a topic of scientific speculation, as well as a subject of philosophical and religious interest, throughout recorded history. The scientific study of dreams is called oneirology.

Dreams mainly occur in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep. At times, dreams may occur during other stages of sleep. However, these dreams tend to be much less vivid or memorable.

Dreams can last for a few seconds, or as long as twenty minutes. People are more likely to remember the dream if they are awakened during the REM phase. The average person has about 3 to 5 dreams per night, but some may have up to 7 dreams in one night. The dreams tend to last longer as the night progresses. During a full 8-hour night sleep, two hours of it is spent dreaming.

In music, a song is a composition for voice or voices, performed by singing. A song may be accompanied by musical instruments, or it may be unaccompanied, as in the case of a cappella songs. The lyrics (words) of songs are typically of a poetic, rhyming nature, though they may be religious verses or free prose.

A song may be for a solo singer, a duet, trio, or larger ensemble involving more voices. Songs with more than one voice to a part are considered choral works. Songs can be broadly divided into many different forms, depending on the criteria used. One division is between "art songs", "pop songs", and "folk songs". Other common methods of classification are by purpose (sacred vs secular), by style (dance, ballad, Lied, etc.), or by time of origin (Renaissance, Contemporary, etc.).

A song is a piece of music for accompanied or unaccompanied voice or voices or, "the act or art of singing," but the term is generally not used for large vocal forms including opera and oratorio. However, the term is, "often found in various figurative and transferred sense (e.g. for the lyrical second subject of a sonata...)." The noun "song" has the same etymological root as the verb "to sing" and the Oxford English Dictionary (OED) defines the word to mean "that which is sung" or "a musical composition suggestive of song." The OED also defines the word to mean "a poem" or "the musical phrases uttered by some birds, whales, and insects, typically forming a recognizable and repeated sequence and used chiefly for territorial defence or for attracting mates."