En banc means all the active judges of a court sitting together to consider and decide a case. Appellate courts can consist of a dozen or more judges, but often they typically cases in panels of three judges. If a case is heard or reheard by the full court, which only occurs a few times a year in each appellate circuit, it is heard en banc.
In some smaller appellate courts, the entire court may consist of only three judges (such as the Texas 12th Circuit Court of Appeals in Tyler), so all cases are automatically heard en banc. On the other hand, the Court of Appeals for the Ninth Circuit is so large that its en banc sittings are not truly so, but only about half of the active judges hear a case, rather than all the active judges.
Statistically, only a tiny percentage of appeals ever go before en banc review. One estimate is that "all federal circuit courts heard only 45 cases en banc out of more than 30,914 cases terminated on their merits since 2010," which is far less than 1%.
In agency proceedings, en banc is an informal meeting held by the agency to hear presentations on specific topics by diverse parties. The Federal Communications Commission, for example, questions presenters and use their comments in considering FCC rules and policies on the subject matter under consideration.