Maria Callas was an American opera singer in the 20th century.
Since he was annoyed by Giuseppina Pentangeli's character after performing "O, soave fanciulla" with her, The Doctor considered singing a duet with Maria Callas the next time. (VOY: "The Swarm")
According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 114), the birth and death years of Maria Callas were 1923 and 1977, respectively. This source said that Ms. Callas "revived classical coloratura roles in the mid-20th century with her lyrical and dramatic versatility."