The examples and perspective in this article may not represent a worldwide view of the subject. (August 2010) (Learn how and when to remove this template message)
A visiting judge is a judge appointed to hear a case as a member of a court to which he or she does not ordinarily belong. In United States federal courts, this is referred to as an assignment "by designation" of the Chief Justice of the United States (for inter-circuit assignments) or the Circuit Chief Judge (for intra-circuit assignments), and is authorized by 28 U.S.C. § 292 (for active district judges) or 28 U.S.C. § 294 (for retired justices and judges).
In many United States Courts of Appeals it is not uncommon for a district judge to sit on a panel as a visiting judge; less frequently it is a judge from another circuit (in active service or, more commonly, in senior status). Retired Supreme Court justices have done the same, including Justices O'Connor and Souter, and very unusually, sitting Justices (in 1984, for example, then-Justice William Rehnquist served as a visiting judge for a jury trial in the United States District Court for the Eastern District of Virginia.). This is sometimes done to ease caseload pressures, and sometimes (as in Rehnquist's case) for experience.
A United States federal district judge's anecdotal description of the designation process: Let's say you are prosecuting or defending a criminal or civil case in your local federal district court, and, out of the blue, your case get reassigned. Not only do you have new judge, but the new judge is a senior status district judge from far away.... How does that happen? Here's a primer.