Once is the fifth studio album by Finnish symphonic metal band Nightwish, released on June 7, 2004 by Nuclear Blast and on November 24 by Spinefarm, with the US release on October 5 by Century Media. In an interview Tuomas Holopainen (main song writer of the band) said that it is up to the listener to find out the meaning of the title Once. Every name has a meaning — though, he doesn't feel comfortable talking about it. This is the last of the Nightwish studio albums to feature vocals by Tarja Turunen.

"Once" utilizes a full orchestra in nine of the eleven songs on the album; unlike "Century Child", Nightwish decided to look for an orchestra outside of Finland this time, choosing the London Philharmonic Orchestra, which also features in their sixth studio album, Dark Passion Play. It is also their second album to feature a full-length song in Finnish, "Kuolema Tekee Taiteilijan". The band added a lot of new elements to their music for this album, as the chorus riff and synthesized drum-beat of Wish I Had an Angel, or the long Native American prayer chant on Creek Mary's Blood. The song Ghost Love Score is an apt example of the combined work done by the orchestra and Nightwish, which the name hints at.

The angel on the album cover is a view of the Angel of Grief, a sculpture by William Wetmore Story. The original is in the Protestant Cemetery of Rome, Italy. The statue shows an angel crying on the tomb of Story's wife; after his death, Story was buried under the same statue, close to his wife. The making of the album cost nearly €250.000, being, at the time, the Finland’s most expensive recording of all time, but it was replaced by Dark Passion Play, also by Nightwish, which spend over €500.000.