Lyrics were phrases set to music.
In 2287, while teaching Leonard McCoy and Spock the campfire song Row, Row, Row Your Boat, James T. Kirk assured them the lyrics were simple. (Star Trek V: The Final Frontier)
In 2369, upon meeting Odo, Lwaxana Troi noted that his name had a lyrical quality to it - that is, that it "rolled off (her) tongue". (DS9: "The Forsaken")
While attempting to sing with a simulation of Giuseppina Pentangeli in 2373, 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 his concern in 2374 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")
In 2376, The Doctor sang the lyric for Mareeza's aria about USS Voyager. (VOY: "Blink of an Eye")
In 2378, he agreed with Seven of Nine that the lyrics of Rock-a-bye Baby were a little grim (Rock-a-bye baby, in the spacedock/When the core blows, the shuttle will rock/When the hull breaks, the shuttle will fall/And down will come baby, shuttle and all.) but that infants responded to the melody, not the words. (VOY: "Human Error")