The Collaborative International Dictionary of English v.0.48:
Bart \Bart.\ n. an abbreviation of baronet. [Abbrev.] "Since he was a baronet he had to be addressed as Sir Henry Jones, Bart." Syn: baronet. [WordNet 1.5]WordNet (r) 3.0 (2006):
Bart 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]