I'll try...

When the government regulates a property to a degree that it effectively deprives the landowner of any productive use of his property, it is considered a taking under the Fifth Amendment.  It is commonly referred to as a regulatory taking.

The portion of the Fifth Amendment that Scott pointed out in his previous post is the source of the government's eminent domain power and in some circumstances it can be invoked without actually divesting the owner of title to the property.

Best,
Robert A. Franco
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