Ten years ago, when I was a student in law school, I learned that it was difficult for a criminal defendant to lose completely his right to assert constitutional defenses. The only way he could relinquish his constitutional defenses, I was told, was by actually "waiving" them. Moreover, in order to establish that a defendant had waived his defenses, the state faced a rigorous test: it had to show, in the famous phrase, that his waiver was "knowing, intelligent, and voluntary." In other words, before the state could permanently prevent a defendant from asserting constitutional defenses, it had to show that he made a deliberate decision to forgo these defenses, that he made the decision after being fully apprised of the consequences and alternatives, and that the state itself had done nothing to make a decision to assert his rights more "costly" than a decision to relinquish them.
Today things are different. Law students now learn that a defendant can lose his constitutional defenses not only by waiving them, but also by "forfeiting" them. The significant difference between waiver and forfeiture is that a defendant can forfeit his defenses without ever having made a deliberate, informed decision to relinquish them, and without ever having been in a position to make a cost-free decision to assert them. Unlike waiver, forfeiture occurs by operation of law without regard to the defendant's state of mind. A good example is the loss of constitutional defenses that occurs by law when a defendant enters a plea of guilty; another is the loss of constitutional defenses that occurs by rule when a defendant fails to assert his defenses in a timely fashion before trial.
Away from Waiver: A Rationale for the Forfeiture of Constitutional Rights in Criminal Procedure,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol75/iss5/15