"one who or that which responds or replies," by 1834 of persons; 1867 of devices (telegraphy); agent noun from respond (v.). Meaning "device which automatically retransmits a pulse or signal" is by 1945.
Entries linking to responder
respond v.
"make answer, give a reply in words," c. 1300, respounden, from Anglo-French respundre, Old French respondere "respond, correspond" and directly from Latin respondere "respond, answer to, promise in return," from re- "back" (see re-) + spondere "to pledge" (see sponsor (n.)). Modern spelling and pronunciation is from c. 1600. From 17c. also "make a liturgical response." Related: Responded; responding.