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Wiktionary
first responder

n. 1 A member of an emergency service who is first on the scene at an emergency 2 A member of an emergency service trained to provide emergency medical care

Wikipedia
First responder

A first responder is an employee of an emergency service who is likely to be among the first people to arrive at and assist at the scene of an emergency, such as an accident, natural disaster, or terrorist attack. First responders typically include police officers, firefighters, paramedics, and emergency medical technicians. A Certified first responder is one who has received certification to provide pre-hospital care in a certain jurisdiction: for example, the Certified First Responder in France. A community first responder is a person dispatched to attend medical emergencies until an ambulance arrives. A Wilderness First Responder is trained to provide pre-hospital care in remote settings, and will therefore have additional skills in ad hoc patient packaging and transport by non-motorized means.

Usage examples of "first responder".

He was a first responder and helped rescue people throughout the day.