Three things are certain in life: death, taxes, and class definitions. Every class-action complaint contains a proposed class definition. It is an indispensable piece of the landscape, just like the case caption, the request for relief, and the signature block. And because it is always present, it is also easy to overlook. Yet, the class definition can be the key to unlocking the mysteries of class certification and may be the most important element of the initial pleading.
On one level, the class definition carries significant practical implications. It tells us who should receive notice of the class action and who will be bound by the judgment. For cases that settle, it tells us who may participate in the settlement. For these reasons, class definitions, as much as any other terms of a complaint, decision, or settlement, circumscribe the rights of absent parties—which, after all, is what representative litigation is supposed to be all about.