Source : Webster's Revised Unabridged Dictionary (1913)

Landgrave \Land"grave`\, n. [G. landgraf; land land + graf earl,
   count; cf. D. landgraaf, F. landgrave.]
   A German nobleman of a rank corresponding to that of an earl
   in England and of a count in France.

   Note: The title was first adopted by some German counts in
         the twelfth century, to distinguish themselves from the
         inferior counts under their jurisdiction. Three of them
         were princes of the empire.

Source : WordNet®

landgrave
     n : a count who had jurisdiction over a large territory in
         medieval Germany