Melody
A melody was a sequence of musical notes that formed a recognizable tune.
In 2366, Jean-Luc Picard suggested that Deanna Troi's hearing music in her mind might be a sign of fatigue, as many people are afflicted with a melody that wouldn't leave their minds. (TNG: "The Survivors")
In 2367, Jono remembered his mother Moira Rossa's singing voice, but could not recall the melody of the song she used to sing. (TNG: "Suddenly Human")
In 2369, Ba'el liked the melody of a Klingon song, but didn't know what the lyrics meant. (TNG: "Birthright, Part II")