A specialized application of a beeper or pager. (aka leash)
Usually used to destroy a weekend for which you had something special planned. It will go off at 4 in the morning no matter what you are doing. More than likely you will be drinking or smoking various substances that make your abilities to respond to a crisis virtually nil.
An offshoot of the emergency pager is the secondary emergency pager which behaves in the same manner as the emergency pager but doesn't have to be dealt with until the third consecutive beep. In many ways this is a worse pager to carry: by the time you need to respond, the problem has either fixed itself or it has ballooned to a problem requiring three more pages, all different from the first set. If the person carrying the emergency pager had actually responded when he/she/it needed to, you would not have to deal with complete morons at 4 in the morning.
Sometimes an emergency pager is one of the downfalls of being a BOFH. However, any smart BOFH will be able to deligate the emergency pager to a lesser administrator or even a tech support person.