Alternative energy is an umbrella term that refers to any source of usable energy intended to supplement or replace fuel sources without the undesired consequences of the replaced fuels. Typically, official uses of the term, such as qualification for governmental incentives, exclude fossil fuels and nuclear energy whose undesired consequences are climate change and difficulties of radioactive waste disposal. Over the years, the nature of what were regarded alternative energy sources has changed considerably, and today because of the variety of energy choices and differing goals of their advocates, defining some energy types as "alternative" is highly controversial.
The term "alternative" presupposes a set of undesirable energy technologies against which "alternative energies" are opposed. As such, the list of energy technologies excluded is an indicator of what problems that the alternative technologies are intended to address. Controversies regarding dominant forms of energy and their alternatives have a long history.
Due to steadily rising gas prices in 2008 with the US national average price per gallon of regular unleaded gas rising above $4.00 at one point, there has been a steady movement towards developing higher fuel efficiency and more alternative fuel vehicles for consumers. In response, many smaller companies have rapidly increased research and development into radically different ways of powering consumer vehicles. While Hybrid Vehicles (such as Toyota's Prius) and Electric Vehicles (such as the Tesla Roadster) are commercially available, other technologies such as fuel cells, flywheels, and ultracapacitors are starting to emerge as viable options.