The Collaborative International Dictionary of English v.0.48:

Bun \Bun\, Bunn \Bunn\, n. [Scot. bun, bunn, OE. bunne, bonne;
   fr. Celtic; cf. Ir. bunna, Gael. bonnach, or OF. bugne tumor,
   Prov. F. bugne a kind of pancake; akin to OHG. bungo bulb,
   MHG. bunge, Prov. E. bung heap, cluster, bunny a small
   swelling.]
   1. any of a variety of slightly sweetened or plain raised
      cakes or bisquits, often having a glazing of sugar and
      milk on the top crust; as, a hot cross bun.
      [1913 Webster +PJC]

   2. a type of coiffure in which the hair is gathered into a
      coil or knot at the top of the head.
      [PJC]

   3. pl. the buttocks. [slang]
      [PJC]

The Collaborative International Dictionary of English v.0.48:

BUN \BUN\ n. [acronym] (Med.)
   same as

   blood urea nitrogen; the concentration of nitrogen in blood
      present in the form of urea; -- used as a measure of
      kidney function.

   Note: Blood usually contains 10 to 15 mg of nitrogen per 100
         ml in the form of urea. --Stedman.
         [PJC]

WordNet (r) 3.0 (2006):

bun
    n 1: small rounded bread either plain or sweet [syn: bun,
         roll]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "bun":
   Danish, Danish pastry, English muffin, Parker House roll,
   Yorkshire pudding, bagel, bialy, bialystoker, braid, chignon,
   clover-leaf roll, coffee cake, coil, crescent roll, croissant,
   cross bun, crumpet, cue, gem, hard roll, hot cross bun,
   kaiser roll, knot, muffin, onion roll, pigtail, pinwheel roll,
   plait, popover, queue, rattail, roll, scone, soft roll, tail,
   topknot, twist