Augmented Backus–Naur form

In computer science, augmented Backus–Naur form (ABNF) is a metalanguage based on Backus–Naur form (BNF) but consisting of its own syntax and derivation rules. The motivation for ABNF is to define an easily usable tool for defining the format of communications protocol payload objects and protocol units.

Source: Wikipedia — Augmented Backus–Naur form (CC BY-SA 4.0)