A deposition is witness's sworn out-of-court testimony. It used to gather information as part of the discovery process and, in limited circumstances, may be used at trial. The witness being deposed is called the "deponent."
Depositions usually do not directly involve the court. The process is initiated and supervised by the parties. Usually, the only persons present at a deposition are the deponent, attorneys for all interested parties, and a person qualified to administer oaths. Sometimes depositions are recorded by a stenographer, although electronic recordings are increasingly common. At the deposition, all parties may question the witness. Lawyers may not coach their clients' testimony, and their ability to object to deposition questions is usually limited.
Depositions are usually hearsay and are thus inadmissible at trial. There are, however, three exceptions to the hearsay rule that are particularly relevant to deposition testimony. The first is when a party admits something in a deposition that is against his or her interest. The second is when a witness's testimony at trial contradicts their deposition. The third is when a witness is unavailable at trial. See Federal Rules of Evidence, Article VIII.
Depositions may also be conducted by written questions. In this kind of deposition, the parties submit questions in advance. At the deposition, the deponent answers those questions and only those questions. Depositions by written questsions are cheaper than depositions by oral questions, because parties' lawyers need not attend, but are much less useful, because it is difficult to follow-up on witness's answers. Usually, parties use interrogatories instead of depositions by written questions.
Deposition rules vary by jurisdiction. See State Civil Procedure Rules. In the Federal Rules of Civil Procedure, Rule 30 governs depositions by oral questions and Rule 31 governs depositions by written questions.
See Civil Procedure.