What is defined as a device that triggers an explosive or starts a chain reaction in munitions?

The term that describes a device which triggers an explosive or initiates a chain reaction in munitions is "detonator." A detonator is designed to provide the necessary energy to initiate an explosive reaction, often used in conjunction with other components like explosives or propellants to ensure a controlled reaction occurs when intended.

While the other options might seem relevant to explosive devices, they have different roles. A fuse typically serves to delay an explosion or control the timing, while an initiator may refer to a component that starts the reaction in a specific way, often in nuclear or high-energy applications. A circuit implies a path through which electricity flows, relevant in electronic detonators but not encompassing the entire function of triggering explosives. Therefore, the correct understanding of a detonator's role highlights its critical function in the operation of munitions, making it the appropriate choice for defining a device that triggers an explosive or starts a chain reaction.