rə̇ˈspändə(r), rēˈs- noun
( -s )
: one that responds:
a. : a person that responds (as to a question, a bid, a kindness)
b. : something that reacts responsively: as
(1) : the main charge of an explosive that requires an initiator to set it off
(2) : the part of a transponder that transmits a radio signal