Notification Appliance Circuit (NAC) - Term used for circuits containing Horns, Strobes, and Speakers.
Initiating Device Circuit (IDC) - Sometimes referred to as a zone, this circuit contains anything that activates the fire alarm system (ex. Pull Stations, smoke detectors, etc..). This is input only, and is an analog signal.
Signaling Line Circuit (SLC) - Term used for the circuit responsible for all devices on the data circuit for Addressable / Intelligent Systems. This circuit can be both input and output because the signal is digital.
Authority Having Jurisdiction (AHJ) - Most of the time this is the fire department that gets to make the decision on certain requirements.
Power Limited - A circuit that has a fixed voltage and amperage, which cannot be exceeded. Power Limited circuits are usually NACs and IDCs.
Non-Power Limited - A circuit that is fused or has a breaker, an example would be the AC power that comes into the panel, or batteries.
Resettable - Refers to devices or circuits that reset to a normal state when the system goes through the resetting process. Resettable Power will temporarily be cut off when the system is reset.
Restorable - Refers to devices that are capable of being reused after they have been activated. Some heat detectors, for example, cannot be restored once tripped.
Trouble - Used to describe anything wrong with the system, such as an open circuit or a ground fault.
Supervisory - A type of signal that is used for non-fire related situations. Examples may include CO Detection and Sprinkler Valve Tamper Switches.
Alarm - Used to describe the entire system or device in an active state. A general fire alarm will cause an evacuation of a building, but some other types of alarm may not.
Power Loop - Sometimes known as an "active" loop, is when a circuit is outputting voltage to the highest potential. For example, this is used when a NAC goes into alarm.
Supervision Loop - Sometimes known as an "inactive" loop, is when a circuit is putting out just enough voltage to ensure there is no open wire or other fault, but not enough voltage to activate a device, such as a notification appliance.