B stands for the Blue Passband, or a Filter implementing that passband. It is one of the passbands/filters of the UBV Photometric System, in which it is centered around Wavelength 445 nm, with a width of 94 nm (Full Width At Half Maximum). It falls in the blue range of the spectrum, its width ranging from violet to green.
The letter is also used to indicate the Magnitude of the signal observed through the filter, e.g., citing "b = 1.5".
AD Leonis (AD Leo)
Algol (Beta Per)
Epsilon Indi (ε Indi)
Globular Cluster Luminosity Function (GCLF)
Sensitivity Function (S)
Stellar Radius Determination
UBV Photometric System