Source : Webster's Revised Unabridged Dictionary (1913)

Duo \Du"o\, n. [It. duo, fr. L. duo two. See {Duet}.] (Mus.)
   A composition for two performers; a duet.

Source : WordNet®

duo
     n 1: two items of the same kind [syn: {couple}, {pair}, {twosome},
           {twain}, {brace}, {span}, {yoke}, {couplet}, {distich},
           {duet}, {dyad}, {duad}]
     2: two performers or singers who perform together [syn: {duet},
         {duette}]
     3: a pair who associate with one another; "the engaged couple";
        "an inseparable twosome" [syn: {couple}, {twosome}, {duet}]
     4: a musical composition for two performers [syn: {duet}, {duette}]
     [also: {dui} (pl)]