Source : Webster's Revised Unabridged Dictionary (1913)

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.]
   A slightly sweetened raised cake or bisquit with a glazing of
   sugar and milk on the top crust.

Source : WordNet®

bun
     n : small rounded bread either plain or sweet [syn: {roll}]