En banc is a legal term of French origin which literally means "on a bench." In a court of law it may refer to a hearing that requires all the judges of a court to be present during a hearing, as opposed to a self-selected panel. This term is interchangeable with in banc, in banco and in bank.
Such a measure is usually only necessary in overly important or complicated cases. Often times the rehearing of an appellate court will be en banc. Appellate courts such as the Supreme Court and other high courts are always en banc.
Of course, only active judges may be present for en banc; those who are ill or otherwise barred for ethical violations are not required.