From The Art and Popular Culture Encyclopedia
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 or an unfavorable witness.