Definition from Nolo’s Plain-English Law Dictionary
At trial, the opportunity to question any witness who testifies on behalf of any other party to the lawsuit (in civil cases) or for the prosecution or other codefendants (in criminal cases). The opportunity to cross-examine usually occurs as soon as a witness completes his or her initial testimony, called direct testimony. Cross-examiners attempt to get the witness to say something helpful to their side, or to cast doubt on the witness's testimony by eliciting something that reduces the witness's credibility -- for example, that the witness's eyesight is so poor that she may not have seen an event clearly. When a witness's direct testimony ends up being hostile to the party that called the witness, sometimes that party's lawyer is allowed to cross-examine his own witness.