The Collaborative International Dictionary of English v.0.48:

Baronet \Bar"on*et\, n. [Baron + -et.]
   A dignity or degree of honor next below a baron and above a
   knight, having precedency of all orders of knights except
   those of the Garter. It is the lowest degree of honor that is
   hereditary. The baronets are commoners.
   [1913 Webster]

   Note: The order was founded by James I. in 1611, and is given
         by patent. The word, however, in the sense of a lesser
         baron, was in use long before. "Baronets have the title
         of 'Sir' prefixed to their Christian names; their
         surnames being followed by their dignity, usually
         abbreviated Bart. Their wives are addressed as 'Lady'
         or 'Madam'. Their sons are possessed of no title beyond
         'Esquire.'" --Cussans.
         [1913 Webster]

WordNet (r) 3.0 (2006):

baronet
    n 1: a member of the British order of honor; ranks below a baron
         but above a knight; "since he was a baronet he had to be
         addressed as Sir Henry Jones, Bart." [syn: baronet,
         Bart]