Define oxidising and reducing agent in terms of oxidation and reduction

Solution:
An oxidizing agent is one that undergoes reduction and in the process oxidizes other species.
A reducing agent is one that undergoes oxidation and in the process reduces other species.

Zn + S $\stackrel{}{\to }$ ZnS
Zn undergoes oxidation from 0 to +2 and hence acts as a reducing agent
S undergoes reduction from 0 to -2 and hence acts as an oxidizing agent.