As explained in Scoping Rules for Variable Bindings, Emacs can optionally enable
lexical binding of variables. When lexical binding is enabled, any
named function that you create (e.g., with defun), as well as
any anonymous function that you create using the lambda macro
or the function special form or the #' syntax
(see Anonymous Functions), is automatically converted into a
closure.
A closure is a function that also carries a record of the lexical environment that existed when the function was defined. When it is invoked, any lexical variable references within its definition use the retained lexical environment. In all other respects, closures behave much like ordinary functions; in particular, they can be called in the same way as ordinary functions.
See Lexical Binding, for an example of using a closure.
Currently, an Emacs Lisp closure object is represented by a list
with the symbol closure as the first element, a list
representing the lexical environment as the second element, and the
argument list and body forms as the remaining elements:
;; lexical binding is enabled.
(lambda (x) (* x x))
⇒ (closure (t) (x) (* x x))
However, the fact that the internal structure of a closure is exposed to the rest of the Lisp world is considered an internal implementation detail. For this reason, we recommend against directly examining or altering the structure of closure objects.