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

concrete syntax

    The syntax of a language including all the
   features visible in the source code such as parentheses
   and delimiters.  The concrete syntax is used when parsing
   the program or other input, during which it is usually
   converted into some kind of abstract syntax tree (conforming
   to an abstract syntax).

   In communications, concrete syntax is called transfer
   syntax.

   (1997-07-21)