Renewable refers to something that can be replenished or restored, and is often used in reference to natural resources such as solar energy, wind power, hydroelectricity, geothermal heat, and biomass. These sources of energy are considered renewable because they can be replenished naturally over a short period of time, unlike non-renewable resources like coal or oil which take millions of years to form and will eventually run out. The term "renewable" is also used in other contexts such as agriculture where it refers to the ability to regrow crops year after year, or in technology where it describes devices that can be recharged or repaired.