Source : Free On-Line Dictionary of Computing

abstract syntax tree
     
         (AST) A data structure representing something which
        has been parsed, often used as a {compiler} or {interpreter}'s
        internal representation of a program while it is being
        optimised and from which {code generation} is performed.  The
        range of all possible such structures is described by the
        {abstract syntax}.
     
        (1994-11-08)