Answer:
oxidation reaction.
Step-by-step explanation:
Every reduction reaction must be accompanied by an oxidation reaction.
Oxidation leads electrons loss whereas reduction implies gain of electrons. So missing electrons should always be the equivalent of obtained electrons. Without something acquiring electrons there can't be any loss. Electrons can't simply disappear!
It implies, but, that oxidation and reduction must occur simultaneously at different locations and the electrons can commute across cables or in a liquid medium by ions.