An offer of proof—also known as an informal bill of exception—is a procedure available during trial for the presentation of evidence the trial court has excluded. An offer of proof is made outside of the jury’s presence and often after the judge has sustained an objection to the admissibility of the evidence.

A bill of exception—also known as a formal bill of exception—is a procedure that allows a party to complain on appeal about evidence that would not otherwise appear in the trial record (because there was no offer of proof made at trial) by detailing the evidence (testimony, etc.) that would have been offered if the trial court had allowed it.

In Massachusetts, an offer of proof is a procedural mechanism used during a trial when the court excludes evidence. The purpose of an offer of proof is to create a record for appellate review by demonstrating what the evidence would have shown if it had been admitted. This process typically occurs outside the presence of the jury, following a judge's ruling that sustains an objection to the evidence's admissibility. The attorney making the offer of proof must articulate, with specificity, the substance of the excluded evidence so that an appellate court can determine whether the trial court's exclusion was erroneous and whether it was harmful. A bill of exception, on the other hand, is a more formalized process that is used to preserve for appeal the record of evidence or objections that were not made during the trial. In Massachusetts, the modern practice has largely replaced the traditional bill of exceptions with the use of a transcript as the official record for appeals. However, the concept remains as a historical reference to procedures for ensuring that an appellate court can review decisions made at trial regarding the admission or exclusion of evidence.