Those forms of energy generated from sources that are inherently regenerative or not “exhaustible” on a “human” time scale - and whose use, by extension, does not endanger natural resources for future generations - qualify as renewable energy.

Consequently, the sun, wind, sea, and heat of the Earth are generally termed “renewable energy sources”. In other words, those sources whose current use will not have a negative effect on their availability in the future. “Non renewable” sources, on the other hand, will be limited in the future both because they are currently consumed at a much higher rate than the rate at which they are formed (especially fossil fuels, such as oil, coal and natural gas) and because they exist in reserves that are not inexhaustible on a human time scale (especially the uranium-235 isotope, the element currently most commonly used to produce nuclear energy).