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defun (short for "define function") is a macro in the Lisp family of programming languages that defines a function in the global environment that uses the form:

(defun <function-name> (<parameter1><parameter2>...<parameterN>) functionbody)

Defining the function addnumbers that adds two numbers:

;; Define a function that adds two numbers together: (defun addnumbers (number1 number2) (+ number1 number2)) (addnumbers 5 4) 9

defining function square that squares a number:

;; Define a function that squares a number: (defun square (x) (* x x))

and defining a function that returns the [[factorial]] of a number:

(defun factorial (number) (if (= number 1) 1 (* number (factorial (- number 1))))) (factorial 6) 720