ENDF stands for "Evaluated Nuclear Data File"
Given that the typical ENDF library is comprised of hundreds of files (approximately one per evaluation), distributed in approximately 15 subdirectories (one per sublibrary), ENDF is not the most accurate name for the library.
Originally the ENDF library was distributed as a single large file either on a magnetic tape or optionally as punch cards. Corrections were mailed as single large files meant to be merged with the original large file. In fact, ENDF releases I-VII were all distributed using this model. The current organization of files has only existed since 2006, when ENDF/B-VII.0 was released.
The "/B" in ENDF/B refers to the "release" branch of our data repository
The library is formally called ENDF/B since originally (in 1968), there were two libraries, ENDF/A and ENDF/B.
ENDF/B is the "release" library. The major release number is given in Roman numerals (e.g. ENDF/B-II). Minor release numbers follow the major release number (e.g. ENDF/B-V.2).
ENDF/A was the "development" area of ENDF/B, a role now taken over by the subversion repository on the NNDC GForge server. The original ENDF/A was mostly taken from the United Kingdom Nuclear Data Library (UKNDL), translated into the ENDF format. (See K. Parker, AWE Report AWRE O.-73/63 (1963))
The ENDF name is also (mis)used to denote the format of the library
The phrase "ENDF format" is, strictly speaking, the correct way to refer to the format in which the ENDF library is distributed. The current version of the format is the ENDF-6 format (although we may want to bump the version number since it is drifting farther from what was used in ENDF/B-VI.8!).