a person who is certified to provide medical care in emergencies before more highly trained medical personnel arrive on the scene: a firefighter trained as a first responder.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for first responder
a person who is trained to provide basic life support in a medical emergency
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012