A lyric was a phrase used for singing along with music.
While attempting to sing with a simulation of Giuseppina Pentangeli, The Doctor forgot the lyrics to the duet they were singing, O, soave fanciulla, due a breakdown in in his program. She described it as an amateur mistake, saying that she had never forgotten the lyrics in her entire career. He called it a momentary lapse, but she demanded another partner. The Doctor, annoyed, then deleted her program. (VOY: "The Swarm")
When Julian Bashir expressed concern that the Dominion was winning the Dominion War, he glibly suggested they could soon be singing "Hail the Conquering Dominion". Jadzia Dax told him "I wouldn't start learning those lyrics just yet, doctor". (DS9: "Favor the Bold")