Definition - What does Adverse Witness mean?
An adverse witness is a witness who deliberately gives a testimony that presents the party that called them to the stand in a negative light. An adverse witness is typically biased against the party that calls for their testimony.
If a judge declares the witness to be an adverse witness, then the attorney may ask leading questions.
Justipedia explains Adverse Witness
Adverse witnesses are witnesses who hold hostile positions against the party who called on them to testify. However, despite the biases of adverse witnesses, their testimonies can still be useful (that is why they are still called).
Adverse witnesses are also referred to as hostile witnesses. If an attorney suspects that a witness is hostile, they can request that the judge declare the person as an adverse witness.