An abstract syntax is a representation of a source code (or data in general) that is independent
of the representation of the program in source code, called the [[concrete syntax]]. 

Typically an abstract syntax for a 
programming language doesn't contain any layout information and comments. 
The set of valid [[abstract syntax trees]] is the abstract syntax of a language.
In the [[Stratego language]] the abstract syntax of a language is described by [[algebraic signatures]].