A medieval form of trial, an ancient way of resolving private disputes.
The two antagonists would fight it out.
If a litigant was a woman, an infant, over 60 or maimed, a champion could be hired to do battle for them.
Brought to England by the Normans, the theory was that God can be the judge of the righteousness of each disputant's cause.
In Life in Norman England, by O. Tomkeieff (London: B. T. Batsford Ltd., 1966), at pp. 131-132, a trial by battle, where the charge was homicide, was described as follows:
"The field was set, with four knights guarding the corners and the spectators, out for a half-day's holiday, sitting or standing in a ring round the outside.
The champions wore padded jerkins, iron caps and wooden staves, possibly with steel tips.
They came on the field hand in hand the one accusing the other of the crime and he retaliating with an accusation of perjury.
Then they had to swear that they would not invoke the aid of demons and evil spirits.
The preliminairies over, at noon they set to work, at first with their staves and, if these were broken with fists, feet or teeth or any part of the anatomy which could be put to use.
It was up to the accuser to get the defendant down before the first star came out, otherwise he lost, and the loser had publicly to proclaim himsself a murderer or perjurer or whatever miscreant he was supposed to be".