In the categorisation adopted for didactic purposes by the classical economists, land is one of three factors of production, the others being labour and capital (economics). It is distinguished from the other two factors by the fact that it exists independently of economic activity, and the fact that its supply is fixed and thus independent of economic activity. The meaning of the term so defined is broader than its meaning in common usage because it includes natural energy sources such as solar energy, tidal energy and wind.
A corollary of fact that its supply is independent of economic activity is the fact that a tax on its value has no effect upon economic activity. As Paul Samuelson puts it "A tax on rent will lead to no distortions or economic inefficiencies". 
- Samuelson and Nordhaus, Economics (1989) pp. 668