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 Arizona, an offer of proof is a procedural mechanism used during a trial when the court excludes evidence. This process allows the attorney to create a record of the evidence that they believe should be admitted, which is done outside the jury's presence. The purpose of an offer of proof is to ensure that the appellate court is aware of the excluded evidence when reviewing the trial court's decision, in case the exclusion is claimed to be erroneous on appeal. On the other hand, a bill of exception, or formal bill of exception, is a more antiquated term and process that was historically used to preserve a record for appellate review when evidence was excluded and no offer of proof was made during the trial. Modern rules of appellate procedure and evidence have largely replaced the need for formal bills of exception, as offers of proof and the record of objections and rulings during the trial generally suffice to preserve issues for appeal. Arizona follows the Federal Rules of Evidence and has similar state rules that govern these procedures.