Abstract syntax refers to the form in which the grammar or rules that govern how a program may be written in a given language are defined, so that they are not tied to a single machine or operating system. In other words, a language's grammar is defined as an "abstract syntax" specification.

What it means, essentially, is that the programming language is fully specified in formal terms so that any arbitrary fragment of code can be unambiguously broken down into the basic building blocks of the language and declared as valid or invalid by the compiler.

This allows anyone who has the specification of the abstract syntax of the language to write a compiler that will correctly compile code written in that language. It also goes some way to giving programmers confidence that a program written for one compiler will also compile with another. This ideal...