The Collaborative International Dictionary of English v.0.48:

Governess \Gov"ern*ess\, n. [Cf. OF. governeresse. See
   Governor.]
   A female governor; a woman invested with authority to control
   and direct; especially, one intrusted with the care and
   instruction of children, -- usually in their homes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

governess
    n 1: a woman entrusted with the care and supervision of a child
         (especially in a private home)

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "governess":
   abbess, chaperon, chatelaine, dame, dowager, duenna, educatress,
   first lady, goodwife, great lady, homemaker, housewife,
   instructress, madam, matriarch, matron, mistress, mother superior,
   schooldame, schoolmarm, schoolmistress, tutoress