In looking for more information about "en banc" I found dictionarty entry which raises another question:
Banc, Bancus, Bank
Banc (?), Ban"cus (?), Bank (?), n. [OF. banc, LL. bancus. See Bank, n.] A bench; a high seat, or seat of distinction or judgment; a tribunal or court. In banc, In banco (the ablative of bancus), In bank, in full court, or with full judicial authority; as, sittings in banc (distinguished from sittings at nisi prius).

What in hell does "nisi prius" mean in this context. Gotta go look that up.