Definition:Backus-Naur Form

Definition
Backus-Naur Form (abbrevated BNF) is a (formal) metalanguage for defining the syntax of a formal language $\mathcal L$.

As such, it is a formal grammar for $\mathcal L$.

BNF is only applicable to formal languages that use the collation system of words and concatenation.

Terminals and Non-Terminals
That is, non-terminals are analogous to grammatical clauses of a natural language, while terminals are analogous to its words.

Also known as
Backus-Naur form was originally called Backus normal form. However, after had worked on refining the work initially done by, his name was added.

Although would prefer it kept its old name, the current name is prevalent.