(define variable expression)
has essentially the same effect as this assignment expression, if variable is bound:
(set! variable expression)
If variable is not bound, however, define binds
variable to a new location in the current environment before
performing the assignment (it is an error to perform a set! on an
unbound variable). If you omit expression, the variable becomes
unassigned; an attempt to reference such a variable is an error.
(define add3
(lambda (x) (+ x 3))) => unspecified
(add3 3) => 6
(define first car) => unspecified
(first '(1 2)) => 1
(define bar) => unspecified
bar error--> Unassigned variable