What is Unearned Increment?

Legal Definition
Unearned increment is an increase in the value of land or any property without expenditure of any kind on the part of the proprietor; it is an early statement of the notion of unearned income. It was coined by John Stuart Mill, who proposed taxing it. Mill's concept was refined and developed by nineteenth-century economist Henry George in his book Progress and Poverty (1879).
-- Wikipedia
Legal Definition
the increase in value that arises from the course of events and not from improvements.