5.3.1 Top level definitions

At the outermost level of a program, a definition

(define ⟨variable⟩ ⟨expression⟩)

has essentially the same effect as the assignment expression

(set! ⟨variable⟩ ⟨expression⟩)

if ⟨variable⟩ is bound to a non-syntax value. However, if ⟨variable⟩ is not bound, or is a syntactic keyword, then the definition will bind ⟨variable⟩ to a new location before performing the assignment, whereas it would be an error to perform a set! on an unbound variable.

(define add3
  (lambda (x) (+ x 3)))
(add3 3)                ⇒ 6
(define first car)
(first '(1 2))          ⇒ 1