Definition from Nolo’s Plain-English Law Dictionary
An error by a judge in the conduct of a trial that an appellate court finds was not damaging enough to the appealing party's right to a fair trial to justify reversing the judgment. Harmless errors include technical errors that have no bearing on the outcome of the trial, and an error that was corrected (such as mistakenly allowing testimony to be heard, but then ordering it stricken and admonishing the jury to ignore it). In general, the more overwhelming the evidence against the appealing party (appellant), the harder it will be to convince the appellate court that any errors were harmful. In such situations, courts rule that even in the absence of the errors, the appellant could not have won.