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.
The Court of Appeals for the Ninth Circuit is so large that its en banc sittings are about half of the active judges, rather than all the active judges.
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.