What is Antejuramentum?

Legal Definition
Antejuramentum, and præjuramentum, historically called juramentum calumniæ (literally, "oath to accuse falsely"), is an oath which both the accuser and accused were obliged to make before any trial or purgation.

The accuser was to swear that he would prosecute the criminal, and the accused was to make oath on the very day that he was to undergo the ordeal, that he was innocent of the claims of which he was charged. If the accuser failed, the criminal was discharged. If the accused failed, he was intended to be guilty, and was not to be admitted to purge himself by the ordeal (see: combat, duel, etc.).
-- Wikipedia
Legal Definition
In Saxon law. A preliminary or preparatory oath, (called also "proejuramentum," and "juramentum calumnies") which both the accuser and accused were required to make before any trial or purgation; the accuser swearing that he would prosecute the criminal, and the accused making oath on the very day that he was to undergo the ordeal that he was innocent of the crime with which he was charged. Whishaw.
-- Black's Law Dictionary
Legal Definition
An oath required of the plaintiff that he would prosecute and of the defendant that he was innocent.
-- Ballentine's Law Dictionary