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> (<parameter-1> <parameter-2> ... <parameter-N>) "optional documentation" <function-body>...) 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)) (square 4) 16