Source : Free On-Line Dictionary of Computing

abstract syntax
     
         A representation of data (typically either a
        message passing over a communications link or a program being
        compiled) which is independent of machine-oriented structures
        and encodings and also of the physical representation of the
        data (called "{concrete syntax}" in the case of compilation or
        "{transfer syntax}" in communications).
     
        A {compiler}'s internal representation of a program will
        typically be specified by an abstract syntax 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)