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

abstract syntax

    A form of representation of data that is
   independent of machine-oriented structures and encodings and
   also of the physical representation of the data.  Abstract
   syntax is used to give a high-level description of programs
   being compiled or messages passing over a communications link.

   A compiler's internal representation of a program will
   typically be an abstract syntax tree.  The abstract syntax
   specifies the tree's structure is specified in terms of
   categories such as "statement", "expression" and
   "identifier".  This is independent of the source syntax
   (concrete syntax) of the language being compiled (though it
   will often be very similar).

   A parse tree is similar to an abstract syntax tree but it
   will typically also contain features such as parentheses which
   are syntactically significant but which are implicit in the
   structure of the abstract syntax tree.

   (1998-05-26)