An aria was a song given by one or two performers in an opera.
In 2373, The Doctor sang the aria O, soave fanciulla from the opera La bohème by Giacomo Puccini. (VOY: "The Swarm")
When B'Elanna Torres experienced a vision of the Klingon afterlife in 2376, The Doctor claimed he'd perform an aria from The Damnation of Faust by Louis Hector Berlioz. (VOY: "Barge of the Dead")
Also that year, Mareeza composed an aria based on the Sky Ship; The Doctor sang the lyric. (VOY: "Blink of an Eye")
In 2377, The Doctor hummed an aria from Rigoletto, which made Fantome relax, so The Doctor subsequently played the full orchestral version. (VOY: "The Void")