The legal right or liability of a person called to testify as a witness in a lawsuit that might be lost or impaired if the party who called him or her to testify wins the case.
This interest against the interest of the party calling a witness to the stand makes him or her an adverse or hostile witness. Although usually the party calling a witness to testify cannot impeach that person's credibility, if the person has an adverse interest, the testimony may be discredited by the party who called that witness to the stand.
n. a right or concern that is contrary to the interest or claim of another.