Answer: A species which can reduce itself and oxidize others
Step-by-step explanation: In a Chemical reaction, there are two species- one is oxidizing agent and the other one is reducing agent.
Oxidizing agent is the substance which reduces its electrons and oxidizes the other species.
Reducing Agent is the substance which oxidizes itself and reduces other.

In this reaction, Zn 2+ is getting reduced in Zn and oxidizing Copper thus acting as an Oxidizing Agent .
And Cu 2+ is reducing itself and oxidizes Zn thus acting as Reducing agent.