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]