defvar

(defvar name [value])

Defines a global variable name, binding it to value only if name is not already bound; if it already has a value, defvar leaves it unchanged (it is idempotent). With no value, the variable is declared but left unbound: until an assignment or a binding gives it a value, a reference to it signals an unbound-variable naming it (boundp tests first). The value is evaluated only when a binding is actually established, and the name symbol is returned.

defvar also proclaims name special: a later let/let* of it -- or a function parameter of that name (defun) -- establishes a dynamic binding (visible to functions called within the extent, restored on exit) rather than a lexical one. See let and progv.