responder

A responder is a person, device, or system that provides a response to a stimulus or request. The term is used in everyday language as well as specialized fields to denote an entity that acts in reply to an input or event.

In emergency management, responders are trained personnel who provide rapid assistance during incidents. This includes police officers, firefighters, emergency medical technicians, and other responders such as search-and-rescue teams. First responders are typically the first to arrive at the scene and begin triage, stabilization, or evacuation as needed.

In computing and information technology, a responder can refer to a component or service that handles requests and produces outputs. In event-driven or service-oriented architectures, response handlers or responders are modules that generate replies or take actions in response to events, queries, or API calls. The term helps distinguish the entity that acts from the data it processes.

In clinical and research contexts, a responder is a patient who shows a measurable improvement or benefit from a treatment, therapy, or intervention. The term is often used alongside non-responders, partial responders, or durable responders, depending on the condition and evaluation criteria. In survey research, responder is sometimes used interchangeably with respondent, though respondent is more common for individuals who answer questions.

The term also appears in other specialized fields, where its meaning reflects the expected reaction to a given stimulus, instruction, or situation.