A secretary who is subordinate to the chief secretary; an assistant secretary; as, an undersecretary of the Treasury.

a secretary immediately subordinate to the head of a department of government

a subordinate and stand-in for a secretary, not normally head of a large operating agency

An undersecretary is a career bureaucrat who typically acts as a senior administrator or second-in-command to a politically-appointed Cabinet Minister or other government official. The title is used in many different political systems.