Definition of Feudum
L. Lat. A feud, fief, or fee. A right of using and enjoying forever the lands of another, which the lord grants on condition that the tenant shall render fealty, military duty, and other services. It is not properly the land, but a right in the land.
This form of the word is used by the feudal writers. The earlier English writers generally prefer the form feodum. There was an older word feum. Its use by the Normans is exceedingly obscure. "Feudal” was not in their vocabulary. Usually it denoted a stretch of land, rarely a tenure or mass of rights. It came to be applied to every person who had heritable rights in land.
That's the definition of Feudum in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.