Once is the fifth studio album by Finnish symphonic metal band Nightwish, released on 7 June 2004 by Spinefarm Records in Finland and Nuclear Blast in the rest of Europe. It is the fifth and last album to feature Tarja Turunen on lead vocals. The album cost nearly €250,000 to make, which made it Finland's most expensive recording ever until the release of Nightwish's next album, Dark Passion Play, which cost over €500,000 to produce. As of 2013, Once had sold 2.3 million copies worldwide, becoming Nightwish's most successful album to date. The remastering of the album was released on 6 August 2021.

Symphonic metal is a cross-generic style designation for the symphonic subsets of heavy metal music subgenres. It is used to denote any metal band that makes use of symphonic or orchestral elements. The style features the heavy drums and guitars of metal with different elements of orchestral classical music, such as symphonic instruments, choirs and sometimes a full orchestra, or just keyboard orchestration.