sexpression

Sexpression, commonly called S-expression or sexp, is a notation for representing nested list data used by Lisp-family languages to express both code and data. It originated with Lisp in the late 1950s and remains a core feature of the language family.

In S-expressions, data is built from atoms (numbers, symbols) and lists enclosed in parentheses. A list is a sequence of elements; for example, (+ 2 3) denotes addition and evaluates to 5 in Lisp. An atom such as x is itself an expression. A quoted form, like '(1 2 3), prevents evaluation.

The reader and the evaluator share the same structure: code is read as a tree of S-expressions and then evaluated. This prefix notation makes Lisp programs highly uniform, because the same syntax is used for both functions and literal data.

Usage and variants: S-expressions are used in Common Lisp, Scheme, Clojure, and other Lisp dialects, as well as in Emacs Lisp. They are also used as a simple, human-readable data format for configuration or data exchange in some contexts.

Notes: The term sexpression is a common variant; the preferred spelling in most references is S-expression. See also Lisp, Homoiconicity, S-expression.