noun

• (music) A musical composition in two parts, each performed by a single voice (singer, instrument or univoce ensemble).
• (music) A song composed for and/or performed by a duo.

verb

• (intransitive) To perform a duet.
  "1822, Lord Byron, Letter to Mr. Moore, Pisa, July 12, 1822, in The Letters of George Gordon Byron, edited by Mathilde Blind, London: Walter Scott, 1887, p. 277, https://books.google.ca/books?id=-DF4Zs_eezUC&printsec=frontcover#v=onepage&q&f=false When you can spare time from duetting, coquetting, and claretting with your Hibernians of both sexes, let me have a line from you."
• (intransitive, zoology, of pairs of animals) To communicate (warnings, mating calls, etc.) through song.
  "1975, Edward O. Wilson, Sociobiology: The New Synthesis, Belknap Press, p. 223, Duetting species are typically monogamous."
• (transitive) To perform (sing, play, etc.) as a duet.
  "Peena and Queena are duetting a giggle-for-giggle […]"
• (transitive) (of two people) To say at the same time, to chorus.
  "“My dear papa!” duetted the girls; but there was something in the husband and father's face, that told the three ladies it would be worse than useless to raise that question at present."