a person or thing that responds.
Electronics. the part of a transponder that transmits the reply.

Origin of responder

First recorded in 1875–80; respond + -er1 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for responder

Contemporary Examples of responder

Word Origin and History for responder

1845 of devices, 1871 of persons, agent noun from respond (v.).

Online Etymology Dictionary, © 2010 Douglas Harper