The Collaborative International Dictionary of English v.0.48:

Motet \Mo*tet"\, n. [F., a dim. of mot word; cf. It. mottetto,
   dim. of motto word, device. See Mot, Motto.] (Mus.)
   A composition adapted to sacred words in the elaborate
   polyphonic church style; an anthem.
   [1913 Webster]

WordNet (r) 3.0 (2006):

motet
    n 1: an unaccompanied choral composition with sacred lyrics;
         intended to be sung as part of a church service; originated
         in the 13th century