Forfeiture and waiver are two concepts that U.S. courts apply in determining whether reversible error has occurred.
Forfeiture is the act of losing or surrendering something as a penalty for a mistake or fault or failure to perform, etc.
If he has merely forfeited the right, e.g. by failing to raise a timely objection, then the standard of review become plain error pursuant to Federal Rule of Criminal Procedure 52(b).
Moreover, in federal cases, the U.S. Court of Appeals may or may not choose to exercise its discretion to correct the plain error.
[1] An example of a waiver would be invited error would be if a defendant requested that the court impose a condition of supervised release.