An electrical circuit is typically referred to as the cable path through which an electrical current flows.
A Loop is a type of electrical circuit where the start and finish of the circuit begin and end at the same location. Most Addressable circuits are Loops (not all) and provide improved integrity of the power and data communications ( the transmission path) between the fire panel and each addressable detector or module.
One of the requirements for addressable circuits is to include a short-circuit isolator to prevent a short circuit on the cable from adversely affecting a significant portion of the devices on the loop.
Where a circuit is NOT a Loop the circuit starts (typically at the fire panel) and ends at a remote location. For a fire detection circuit it will normally end with an End of Line Resistor ("EOL"). A circuit of this type is often referred to as a spur or line.