The top level loop is the Common Lisp mechanism by which the user normally interacts with the Common Lisp system. This loop is sometimes referred to as the Lisp read-eval-print loop because it typically consists of an endless loop that reads an expression, evaluates it and prints the results.
The top level loop is not completely specified; thus the user interface is implementation-defined. The top level loop prints all values resulting from the evaluation of a form. The next figure lists variables that are maintained by the Lisp read-eval-print loop.