Some sources for energy are called alternative sources. To what sources of energy are they an alternative?

Alternative energy sources are typically alternatives to fossil fuels, such as coal, oil, and natural gas, which are conventional energy sources. They include renewable options like solar, wind, geothermal, hydroelectric, and biomass energy. These alternatives aim to reduce greenhouse gas emissions, decrease dependence on finite resources, and promote environmental sustainability. By harnessing natural processes, alternative energy sources provide cleaner and more sustainable energy solutions.