An expression consisting of a variable is a variable reference. The value of of the variable reference is the value stored in the location to which the variable is bound. It is an error to reference an unbound variable.
Example:
[1] (define variable "this is a variable")
variable
[2] variable
"this is a variable"
[3] (define unassigned #u)
unassigned
[4] unassigned
ERROR!
message : variable unassigned
irritant : unassigned
expression: unassigned
continue : possible, continue with value as obtained from variable
Error (? for help) >> ^D
[5] undefined
ERROR!
message : variable not found
irritant : undefined
expression: undefined
continue : possible, continue with value as obtained from variable
Error (? for help) >> ^D
[6]