A song had at least a chord, a set of lyrics and some music, to which those lyrics were sung, or at least read to. (PROSE: Oh No It Isn't!)
Oh, I know this great song about this bloke and his girlfriend. She drops the ring he gives her on the railway track, and when he goes back to get it, she's killed by the train and he's really miserable for the rest of his life.
Culture
On Terra Alpha it was the law that everyone be happy, including their songs; these needed to be "songs about sunshine and furry animals" and not about the "blues". (TV: The Happiness Patrol)
Both the Tenth (TV: Journey's End) and Eleventh Doctors (TV: The Beast Below) claimed that people with whom he and his companions had interacted with sang or would write songs about them following their meetings. The Tenth Doctor claimed, after returning Donna Noble to her mother and grandfather, that "there are people living in the light, and singing songs of Donna Noble, a thousand million light years away". (TV: Journey's End)
Cultural songs
Several cultures and species had specific groups who passed on tales through song. The Menoptera spoke to Barbara Wright of "song-spinners" who sang of the beauty of the Temple of Light. (TV: The Web Planet) Similarly Avram was a travelling singer – a songsmith, who travelled around Mesopotamia telling his stories. (PROSE: Timewyrm: Genesys)
Several species and individuals had or received songs that were both telepathic and prophetic. The Ood sang songs like this, (TV: Planet of the Ood, The End of Time) whilst Saul intoned and projected messages as songs. (PROSE: Timewyrm: Revelation)
The Sisterhood of Karn had what Solon termed a "song of death" for when they began to make ready to sacrifice the Fourth Doctor to the sacred flame. (TV: The Brain of Morbius)
On New Earth in New New York on the Motorway, the people trapped in the decades long traffic jam held their society and faith together through the songs and hymns. (TV: Gridlock)
The Ice Warriors had several songs of the Old Times, the songs of the Red Snow. (TV: Cold War)
Other cultures used songs to comment on society, such as Plasticine's "Dissent Is Good", a song about freedom of speech and action, sung by and composed with the assistance of Bernice Summerfield. (PROSE: No Future)
Whilst on Tyler's Folly the SSSSSSS Nazi troopers sang obscene songs about Austrian milkmaids in groups around the canned campfires and drank plastiflagons of Saknusseinm Pils. (PROSE: Down)
The Sittuun did not have songs or even music. Charlie liked human music. (PROSE: Night of the Humans)
Language
"Song" could also be used as a term for an oft repeated phrase or style. The Master mocked the Third Doctor, saying, "Your usual song of death and disaster? I do wish you'd learn a new tune," as the Master prepared to unleash Kronos on the Doctor. (TV: The Time Monster)
It could also be used to signal a change in opinion to "sing a different song", as Styggron claimed of Guy Crayford when he objected to the killing of the Fourth Doctor. (TV: The Android Invasion)
Similarly it could also be used to denote a small sum, that something like a house could be sold for a small amount – "a song". (PROSE: Who Killed Kennedy)
Alternatively some languages, such as those of the Gamma Forests, used "song" in place of similar words like "melody". (TV: A Good Man Goes to War)
Certain species spoke in a language of song; the Krakenites had songs of identity, interpreted by non-Krakenites as a series of clicks and booms. (PROSE: Dreamstone Moon)
The Twelfth Doctor revealed that babies thought that laughter was singing. (TV: The Girl Who Died)
Songs and language
The Master also liked to quote lyrics of songs to people, idly commenting to Jo (while she was his prisoner), "Miss Grant, you had better prepare yourself for a long stay. I'm afraid it's not a very comfortable place, but as the old song says, 'Be it ever so humble, there's no place like home.'" (TV: Frontier in Space)
Likewise, Cassandra O'Brien.Δ17 taunted the guests on Platform One who were trapped about to be burnt and killed, "How did that old Earth song go? 'Burn, baby, burn'." (TV: The End of the World)
Biological
Chimeron babies made noises that were part-song, part-defence mechanism against the Bannermen. (TV: Delta and the Bannermen)
Harrison Chase believed that plants respondeded to music, composing a song for plants especially for them. (PROSE: Doctor Who and the Seeds of Doom)
Emotion
Vincent van Gogh could see a sadness in Amy, saying. "Oh, Amy. I hear the song of your sadness. You've lost someone, I think", seeing Amy Pond's loss of Rory, even if she at the time did not know herself. (TV: Vincent and the Doctor)