error in a trial which is significantly sufficient to cause the entire trial to be reversed or a new trial to be granted by a reviewing appellate court
A procedural error during a trial or hearing harmful enough to reverse the judgment of a lower court.
Reversible error is an error by the trier of law (judge) or the trier of fact (jury - or again the judge if it is a bench trial) or malfeasance by one of the trying attorneys which results in an unfair trial. It is to be distinguished from harmless error that do not rise to a level that bring the validity of the judgment into question and thus do not lead to a reversal upon appeal.