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(guile.info)About Environments

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Names, Locations, Values and Environments
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We said earlier that a variable name in a Scheme program is associated
with a location in which any kind of Scheme value may be stored.
(Incidentally, the term "vcell" is often used in Lisp and Scheme
circles as an alternative to "location".)  Thus part of what we mean
when we talk about "creating a variable" is in fact establishing an
association between a name, or identifier, that is used by the Scheme
program code, and the variable location to which that name refers.
Although the value that is stored in that location may change, the
location to which a given name refers is always the same.

We can illustrate this by breaking down the operation of the `define'
syntax into three parts: `define'

   * creates a new location

   * establishes an association between that location and the name
     specified as the first argument of the `define' expression

   * stores in that location the value obtained by evaluating the second
     argument of the `define' expression.

A collection of associations between names and locations is called an
"environment".  When you create a top level variable in a program using
`define', the name-location association for that variable is added to
the "top level" environment.  The "top level" environment also includes
name-location associations for all the procedures that are supplied by
standard Scheme.

It is also possible to create environments other than the top level one,
and to create variable bindings, or name-location associations, in those
environments.  This ability is a key ingredient in the concept of
closure; the next subsection shows how it is done.