ANSI Common Lisp 3 Evaluation and Compilation 3.1 Evaluation 3.1.1 Introduction to Environments
220.127.116.11 Environment Objects
Some operators make use of an object,
called an environment object,
that represents the set of lexical bindings needed to perform
semantic analysis on a form in a given lexical environment.
The set of bindings in an environment object
may be a subset of the bindings that would be needed to actually
perform an evaluation; for example, values associated with
variable names and function names in the corresponding
lexical environment might not be available in an environment object.
The type and nature of an environment object is implementation-dependent.
The values of environment parameters to macro functions
are examples of environment objects.
The object nil when used as an environment object
denotes the null lexical environment;
see Section 18.104.22.168.1 The Null Lexical Environment.