Panic button

A panic alarm is an electronic device designed to assist in alerting somebody in emergency situations where a threat to persons or property exists.
A panic alarm is frequently but not always controlled by a concealed panic alarm button. These buttons can be connected to a monitoring center or locally via a silent alarm or an audible bell/siren. The alarm can be used to request emergency assistance from local security, police or emergency services. Some systems can also activate closed-circuit television to record or assess the event.
Many panic alarm buttons lock on when pressed, and require a key to reset them.
Alarm.
Examples of alarm panic buttons are:
Medical alert.
A medical alert panic button or medical alarm is an electronic device worn on a bracelet or necklace as part of a medical alert system. When pressed, it sends a wireless signal to a home console which dials alarm monitoring staff and alerts them of an emergency condition. Depending on the severity of the situation, alarm monitoring staff will summon friends, family, or emergency services. A panic button alarm is a self-contained electronic device powered by an internal long-life battery, typically Waterproof and designed to be shock resistant and highly durable.
Holdup alarms.
Holdup alarms are alarms that require a person under duress to covertly trigger the alarm to summon the proper authorities. These types of alarms are most commonly found in retail establishments and financial institutions, but are sometimes an integrated feature of home burglar alarms. The trigger could be a push button, electronic money clip, a foot rail, or a number of other things. Either the person under duress or a witness can activate this kind of alarm. For example, if someone is ambushed outside of their home and told to disable their alarm system they can possibly enter a special duress code that is different from their normal deactivation code to notify authorities without arousing suspicion. These alarms are almost always silent and usually require a manual reset with a key or a special code.
Monitoring services.
The monitoring service (central station) is a call center facility that is staffed at all times to receive calls from the system console. Monitoring service centers that are approved by Underwriters Laboratories (UL) have internal backup systems to add redundancy. Some monitoring services employ trained operators enabling them to better 
evaluate the severity of help requests. In most less developed countries however, response to panic alarms are slow.
MIDI.
In a MIDI instrument when the note-off message for a played note is not received, the note will sound on endlessly, and also has the potential to rise in amplitude enough to damage the speakers or other components in the sound system. Hitting the panic button will send a note-off command to all keys, stopping any notes that were still playing.