"Eclipse" is the tenth and final track from English rock band Pink Floyd's 1973 album, The Dark Side of the Moon. It was written and sung by Roger Waters, with harmonies by David Gilmour and Rick Wright. After Waters left the band, Gilmour sang the lead vocal when performing live.

On the album, the song transitions, without noticeable break, from the previous song, "Brain Damage", and the two are often played together as a single track on the radio (some DJs call the combined track "The Dark Side of the Moon"). The end of the track consists of a fading heartbeat, identical to the opening of the first track on the album, "Speak to Me".

This song serves as the album's end and features a loud, repetitive melody that builds up, then ends with a very quiet outro. When the main instrumentation ends at 1:30, the sound of a heartbeat from the first track, "Speak to Me", appears, which appears again in 9/8, and gradually fades to silence.

Harmonically, the song consists of a repeating 4-bar chord progression: D, D/C, B♭maj7, and A7sus4 resolving to A7. The bass line is a descending tetrachord.

David Gilmour recorded two tracks of rhythm guitar, playing arpeggios, one in open position, and one much higher, around the tenth fret. The lower-pitched guitar part includes the open G and E strings during the B♭maj7, resulting in an added sixth and a dissonant augmented fourth. The quartet of backing singers vary their parts, rising in volume, and echoing some of Roger Waters' vocals, as the piece builds in intensity. On the last repetition of the chord progression, the B♭maj7 leads directly to a climax on D major, resulting in a "brightening" effect (known as the Picardy third), as the aforementioned implication of D minor in the B♭maj7 chord shifts to the major.

Waters wrote the lyrics on the road for the "Brain Damage" / "Eclipse" closing sequence as he felt the whole piece was "unfinished". The final words sung on the song and, indeed the album The Dark Side of the Moon, directs the listener, "and everything under the sun is in tune, but the sun is eclipsed by the moon." Waters explained the meaning of these words as well as the entire song by asserting:

I don't see it as a riddle. The album uses the sun and the moon as symbols; the light and the dark; the good and the bad; the life force as opposed to the death force. I think it's a very simple statement saying that all the good things life can offer are there for us to grasp, but that the influence of some dark force in our natures prevents us from seizing them. The song addresses the listener and says that if you, the listener, are affected by that force, and if that force is a worry to you, well I feel exactly the same too. The line 'I'll see you on the dark side of the moon' is me speaking to the listener, saying, 'I know you have these bad feelings and impulses because I do too, and one of the ways I can make direct contact with you is to share with you the fact that I feel bad sometimes.

The doorman of Abbey Road Studios, Gerry O'Driscoll, is heard speaking at 1:37, answering the question: "What is 'the dark side of the moon'?" with: "There is no dark side in the moon, really. Matter of fact, it's all dark. The only thing that makes it look light is the sun."