Voluntary Oath
Defined in 2 dictionaries — Cyclopedic (1922), Black's (1910)
The Cyclopedic Law Dictionary
Walter A. Shumaker and George Foster Longsdorf; ed. James C. Cahill · 1922
An oath taken in some extrajudicial matter, or before some magistrate or officer who cannot compel it to be taken.
Black's Law Dictionary
Henry Campbell Black, M.A. · 1910
Such as a person may lake in extrajudicial matters, and not regularly in a court of justice, or before an officer invested with authority to administer the same. Brown
Defined under Oath in Black's Law Dictionary.