Adverse Witness

witness that presents disadvantageous evidence, witness that provides evidence that helps the other side of a court case (Legal)

In United States law, a hostile witness is a witness in a trial who testifies for the opposing party or a witness who offers adverse testimony to the calling party during direct examination. A witness called by the opposing party is presumed hostile. A witness called by the direct examiner can be declared hostile by a judge, at the request of the examiner, when the witness' testimony is openly antagonistic or clearly prejudiced to the opposing party. A party examining a hostile witness may question the witness as if in cross-examination, thus permitting the use of leading questions. A hostile witness is sometimes known as an adverse witness.
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Noun
1. a witness whose relationship to the opposing party is such that his or her testimony may be prejudiced against the opposing party; "a hostile witness can be asked leading questions and cross-examined"
(synonym) hostile witness
(hypernym) witness

The right of a party calling a witness to show that the witness made a prior inconsistent statement. - (read more on Adverse Witness)