What does the 'B' parameter represent in Wikipedia's page on window functions?
See paragraph from wiki. There is an explanation of B metric:


  Each figure label includes the corresponding noise equivalent bandwidth metric (B),[note 1] in units of DFT bins.


Note 1:


  Mathematically, the noise equivalent bandwidth of transfer function H is the bandwidth of an ideal rectangular filter with the same peak gain as H that would pass the same power with white noise input. In the units of frequency f (e.g. hertz), it is given by:
  $$
B_{noise}={\frac {1}{|H(f)|_{max}^{2}}}\int _{0}^{\infty }|H(f)|^{2}df. 
$$