<language, data> 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.