In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming language Lisp, which uses them for source code as well as data. In the usual parenthesized syntax of Lisp, an S-expression is classically defined as 1. * an atom, or 2. * an expression of the form (x . y) where x and y are S-expressions. (x y z) stands for (x . (y . (z . NIL)))