Common Lisp the Language, 2nd Edition
A loop is a series of expressions that are executed one or more times, a process known as iteration. The Loop Facility defines a variety of useful methods, indicated by loop keywords, to iterate and to accumulate values in a loop.
Loop keywords are not true Common Lisp keywords; they are symbols that
are recognized by the Loop Facility and that provide such capabilities
as controlling the direction of iteration, accumulating values inside
the body of a loop, and evaluating expressions that precede or follow
the loop body. If you do not use any loop keywords, the Loop Facility
simply executes the loop body repeatedly.