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 New York, an offer of proof is a procedural tool used during a trial when the court excludes evidence. It allows the attorney to create a record of what the evidence would have shown, had it been admitted. This is done outside the presence of the jury, typically after an objection to the evidence has been sustained by the judge. The purpose of an offer of proof is to preserve the issue for appeal by demonstrating how the excluded evidence could have affected the outcome of the case. On the other hand, a bill of exceptions, also known as a formal bill of exception, is used to preserve for appellate review any error regarding excluded evidence when no offer of proof was made at trial. It provides a written record of the excluded evidence, including testimony or documents, that the party contends should have been admitted. Both procedures are essential for ensuring that a party's right to appeal is protected, particularly in cases where the exclusion of evidence might be argued to have been erroneous and prejudicial.