Common Lisp the Language, 2nd Edition

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15. Lists

A cons, or dotted pair, is a compound data object having two components called the car and cdr. Each component may be any Lisp object. A list is a chain of conses linked by cdr fields; the chain is terminated by some atom (a non-cons object). An ordinary list is terminated by nil, the empty list (also written ()). A list whose cdr chain is terminated by some non-nil atom is called a dotted list.

The recommended predicate for testing for the end of a list is endp.