"En Banc" refers to the complete "bench" of a court. If a case is heard en banc, this means that the complete set of judges will sit, hear the case, and participate in the decision. The term appears in the legal code as "in banc." Cases processed in this fashion are usually considered more important or more complex.
In the United States, en banc is used most frequently in conjunction with Appeals Courts, which contain many judges but usually process cases in smaller panels of three judges. The United States Tax Court, which normally assigns one judge to each case, also sometimes sits en banc for particularly important cases. The Supreme Court could be said to hear all of its cases en banc; there is no procedure for using a smaller panel.
The United States Court of Appeals for the Ninth Circuit is an exception to the above. It contains a total of 28 judges, who very rarely hear a case all together. More commonly, an en banc panel for this circuit contains 11 randomly chosen judges rather than the usual three. This exception to procedure is legal, according to the laws of Congress, for courts with more than 15 members; however, it does play a role in criticisms that the Ninth Circuit court is too large.
A court will adopt the en banc procedure for cases that seem to have unusual importance. This importance can stem from the subject matter of the case, or from a court's felt need to express a uniform opinion. The determination of whether a case meets one of these criteria must be made by a majority of the circuit judges. A party in a particular case may suggest that it be heard en banc, but there is no guarantee that the court will respond.
Short of an appeal to the Supreme Court, only an en banc court has the power to overrule a decision by a panel in the same circuit. Thus, an en banc appeal is often a step on the pathway of an important case from its original jurisdiction to the Supreme Court. Once a three-judge panel appeals court rules, the losing side can file an appeal to be heard in the same circuit en banc. There are, however, several mechanisms through which a case can pass directly to the Supreme Court.