The Collaborative International Dictionary of English v.0.48:

Burglar \Bur"glar\, n. [OE. burg town, F. bourg, fr. LL. burgus
   (of German origin) + OF. lere thief, fr. L. latro. See
   Borough, and Larceny.] (Law)
   One guilty of the crime of burglary.
   [1913 Webster]

   Burglar alarm, a device for giving alarm if a door or
      window is opened from without.
      [1913 Webster]

WordNet (r) 3.0 (2006):

burglar
    n 1: a thief who enters a building with intent to steal

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BURGLAR. One who commits a burglary. (q. v.)