— Special Form: defun name argument-list body-forms

defun is the usual way to define new Lisp functions. It defines the symbol name as a function that looks like this:

          (lambda argument-list . body-forms)

defun stores this lambda expression in the function cell of name. It returns the value name, but usually we ignore this value.

As described previously (see Lambda Expressions), argument-list is a list of argument names and may include the keywords &optional and &rest. Also, the first two forms in body-forms may be a documentation string and an interactive declaration.

There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. See Symbol Components.

Here are some examples:

          (defun foo () 5)
               ⇒ foo
          (foo)
               ⇒ 5
          
          (defun bar (a &optional b &rest c)
              (list a b c))
               ⇒ bar
          (bar 1 2 3 4 5)
               ⇒ (1 2 (3 4 5))
          (bar 1)
               ⇒ (1 nil nil)
          (bar)
          error--> Wrong number of arguments.
          
          (defun capitalize-backwards ()
            "Upcase the last letter of a word."
            (interactive)
            (backward-word 1)
            (forward-word 1)
            (backward-char 1)
            (capitalize-word 1))
               ⇒ capitalize-backwards

Be careful not to redefine existing functions unintentionally. defun redefines even primitive functions such as car without any hesitation or notification. Redefining a function already defined is often done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.

— Function: define-function name definition
— Function: defalias name definition

These equivalent special forms define the symbol name as a function, with definition definition (which can be any valid Lisp function).

The proper place to use define-function or defalias is where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is because define-function and defalias record which file defined the function, just like defun. (see Unloading).

By contrast, in programs that manipulate function definitions for other purposes, it is better to use fset, which does not keep such records.