Backus-Naur form, a human-reader friendly convention for writing context free grammars. It incorporates notations for things like optionality and repetition, which are strictly speaking redundant (and so omitted from YACC notation), but which humans like to see. In (one of) its extended forms (E-BNF), used for a human-friendlier statement of the YACC formal grammar of Lojban. In cases of conflict, however, it is the YACC rather than the E-BNF form of the grammar that is considered canonical.
There are easily dozens of different 'Extended' BNFs, btw.