In order to define alternative energy we first need to define alternative energy sources. Alternative energy source is any source of energy that does not belong to traditional energy sources, namely fossil fuels (coal, oil and natural gas). Some dictionaries like for instance Oxford dictionary puts alternative energy sources in correlation to environment and states that term alternative energy sources refers to energy sources that do not use up natural resources or harm the environment.
There have been many controversies about this term and even today defining alternative energy sources will often be connected with plenty different second opinions. For instance nuclear energy is thought by some to be alternative source of energy while other say that only renewable energy sources are in fact alternative energy sources. The same situation is with hydropower because some people think that hydropower is a traditional source of energy together with fossil fuels.
To stay out of these controversies as much as possible I will only tell you a few words about the most common alternative energy sources: solar energy, wind energy and geothermal energy. Other alternative energy sources include the likes of biomass and hydrogen.
Solar energy is coming from the Sun and this is the most abundant source of energy available on our planet. Solar power industry still depends on subsidies and harnessing solar energy is still connected with intermittency issue (since Sun doesn't shine all day long). However given the enormous potential of solar energy as well as huge funding and numerous researches on this matter it is pretty realistic to say that one day solar energy will become top energy source in the world.
Wind energy is better in cost-competitiveness when compared to solar energy but still suffers from intermittency issue just like solar does. Many countries have started massive wind energy expansion (most notably China) and in years to come it is expected that wind farms will move offshore where there are more powerful and more frequent ocean winds.
Geothermal energy refers to the heat stored within the earth's core. Geothermal energy is unlike solar and wind available 24-7 but has high drilling costs, which means that with current technological development harnessing geothermal energy is only viable near tectonic plates. This is also the reason why there are only 24 countries in the world currently harnessing geothermal energy.
These three alternative energy sources have one big advantage over traditional fossil fuels, and that is their environmentally friendly character. Fossil fuel burning is main contributor to climate change and air pollution which means that world needs to replace fossil fuels with alternative energy sources as soon as possible in order to avoid horrific climate change scenario.