"Halcyon" is a song written and performed by Orbital, dedicated to the Hartnolls' mother, who was addicted to the tranquiliser Halcion (Triazolam) for many years. The song features two samples from earlier international hit singles. The first sample is a backmasked vocal sample by Kirsty Hawkshaw from "It's a Fine Day" by Opus III (1992). Ed Barton, the composer of "It's a Fine Day", receives a co-writing credit for the track. The second is a vocal harmonies sample in the song "Leave It" from the 1983 album 90125 by the progressive rock band Yes. There is no co-writing credit for the Yes sample.

This original form of Halcyon is relatively uncommon; it first appeared on the Radiccio EP, and was only in the U.S. release as a single under its own name. It did not appear on a full-length album until the compilation Work 1989-2002, and then only in its shorter "single edit".

On Orbital's second eponymous album the track "Halcyon + on + on" appears, a slightly more upbeat and melodic remix of the original song. In contrast to the original, the remix is far more widely known, and has been featured on several movie soundtracks (most notably Mortal Kombat, Hackers, CKY2K and Mean Girls). The title of the remix is inspired by a contemporary advertising slogan used by the Ariston washing machine company ("Ariston + on + on"). "Halcyon + on + on" is somewhat shorter than the original, at 9 minutes and 27 seconds long.

"Halcyon" is one of the mainstays of Orbital's live performances, in which it is frequently remixed with clips from Belinda Carlisle's "Heaven is a Place on Earth" and Bon Jovi's "You Give Love a Bad Name". A 1999 performance containing these samples appears on the album Orbital: Live at Glastonbury 1994–2004. On occasion (notably the group's 2004 "last ever" performance broadcast on BBC Radio 1) it has also incorporated the chorus of "I Believe in a Thing Called Love" by The Darkness.

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