The Free On-line Dictionary of Computing (30 December 2018):

context-free grammar
CFG

    (CFG) A grammar where the syntax of each constituent
   (syntactic category or terminal symbol) is independent of the
   symbols occuring before and after it in a sentence.  A
   context-free grammar describes a context-free language.

   Context-free grammars can be expressed by a set of "production
   rules" or syntactic rules.  For example, a language with symbols
   "a" and "b" that must occur in unequal numbers can be represented
   by the CFG:

    S → U | V
    U → TaU | TaT | UaT
    V → TbV | TbT | VbT
    T → aTbT | bTaT | ε

   meaning the top-level category "S" consists of either a "U" or a
   "V" and so on.  The special category "ε" represents the empty
   string.  This grammar is context-free because each rule has a
   single symbol on its left-hand side.

   Parsers for context-free grammars are simpler than those for
   context-dependent grammars because the parser need only know the
   current symbol.

   Algol was (one of?) the first languages whose syntax was
   described by a context-free grammar.  This became a common
   practice for programming languages and led to the notation for
   grammars called Backus-Naur Form.

   (2014-11-24)