- 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 parenthesized syntax of Lisp, an S-expression is classically defined as an atom of the form x, or as an expression of the form (x . y), where x and y are S-expressions. ← Wikipedia