The Collaborative International Dictionary of English v.0.48:

Frau \Frau\ (frou), n.; pl. Frauen (frou"en). [G. Cf. 1st
   Frow.]
   In Germany, a woman; a married woman; a wife; -- as a title,
   equivalent to Mrs., Madam.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Frau
    n 1: a German courtesy title or form of address for an adult
         woman