Definition - What does Supernumerary Witness mean?
A supernumerary witness is a witness that is extra, or surplus to the required amount of witnesses. For example, if three witnesses are needed for the signing of an official legal document, and there are four, the fourth would be the supernumerary witness.
Supernumerary witnesses simply add extra strength to the witnessing of an event.
Justipedia explains Supernumerary Witness
Often, witnesses are needed in order for certain legal proceedings to take place. Witnesses are simply there to watch the event happen and then declare that the event did in fact happen. This is to prevent fraud or prevent prevent people from lying about events that did not actually take place. For example, witnesses can be needed for marriages or for the signing of wills.
When extra witnesses are present in addition to the required number, then they are supernumerary witnesses.