Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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