8.2.1 Variables and Regions
Any identifier that is not a syntactic-keyword may be used as a variable. A variable may name a value.  A variable that does so is said to be bound to the value.  The set of all visible bindings in effect at some point is known as the environment in effect at that point. The value to which a variable is bound is called the variable's value.
Certain expression types are used to bind variables to new values. The most fundamental of these binding constructs is the lambda expression, because all other binding constructs can be explained in terms of lambda expressions.  The other binding constructs are let, let*, and letrec expressions.
Like Algol and Pascal, and unlike most other dialects of Lisp except for Common Lisp, the expression language is a statically scoped language with block structure.  To each place where a variable is bound in an expression there corresponds a region of the expression text within which the binding is effective.  The region is determined by the particular binding construct that establishes the binding; if the binding is established by a lambda expression, for example, then its region is the entire lambda expression.  Every reference to,  or assignment of, a variable refers to the binding of the variable that established the innermost of the regions containing the use.  If there is no binding of the variable whose region contains the use, then the use refers to the binding for the variable in the top-level environment, if any; if there is no binding for the identifier, it is said to be unbound.