The Collaborative International Dictionary of English v.0.48:

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

WordNet (r) 3.0 (2006):

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