[IBM] The error code displayed on line 25 of a 3270 terminal (or a PC emulating a 3270) for various kinds of protocol violations and “unexpected” error conditions (including connection to a down computer). On a PC, simulated with ‘->/_’, with the two center characters overstruck.

Note: to appreciate this term fully, it helps to know that “broken arrow” is also military jargon for an accident involving nuclear weapons....