The Collaborative International Dictionary of English v.0.48:

Bk \Bk\ n.
   the chemical symbol for berkelium.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Bk
    n 1: a radioactive transuranic element; discovered by bombarding
         americium with helium [syn: berkelium, Bk, atomic
         number 97]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

BK
       Buero Kommunikation