PLAIN ERROR

rule applicable to appellate courts that requires the reversal of a conviction and the award of a new trial where an obvious error in the trial proceedings affecting the fundamental right of the accused to a fair trial was not objected to at the time it occurred and went uncorrected by the trial court. EXAMPLE: The prosecutor introduces very prejudicial evidence at Roy’s trial. The judge fails to instruct the jury to limit their consideration of that evidence, despite the obvious need for such an instruction. Roy is convicted and the case is appealed. Even though Roy’s attorney did not object to the introduction at the time it occurred – a procedure that would normally be required before a new trial could be granted – the appellate court may apply the plain error rule and grant Roy a new trial. Compare harmless error; miscarriage of justice.
Source: Barron's Dictionary of Legal Terms, Steven H. Gifis, 5th Edition; © 2016

"The second limitation on appellate authority under Rule 52(b) is that the error be "plain." "Plain" is synonymous with "clear" or, equivalently, "obvious.""

- USA v. Olano, 507 US 725 (1993)