Source : Webster's Revised Unabridged Dictionary (1913)

Undersecretary \Un`der*sec"re*ta*ry\, n.
   A secretary who is subordinate to the chief secretary; an
   assistant secretary; as, an undersecretary of the Treasury.

Source : WordNet®

undersecretary
     n : a secretary immediately subordinate to the head of a
         department of government