Epoch B1950.0 is a standard point in time used as a reference in astronomy, i.e., to specify the date and time for a set of coordinates on the Celestial Sphere. It was commonly used before the currently preferred epoch, Epoch J2000.0. It is precisely defined, the exact point being somewhat before midnight, December 31, 1949 GMT, specifically, Julian Date 2433282.4235.
The "B" indicates that it is a Besselian Epoch, based upon a precise definition of the solar year, the Besselian Year, which is now considered imperfect. Earlier Besselian Epoches used in astronomy include B1900.0, B1925.0, and B1875.0.