Alternative energy is an broad term generally used to describe any type of usable energy that can replace existing fuel types but without the undesired consequences of the existing fuels.
“Official” use of the term, such as for qualification for governmental incentives, excludes fossil fuels and nuclear energy whose undesired consequences relate to greenhouse gases, climate change and the problem of disposing of radioactive waste.
The nature and consept of what had previously been regarded as alternative energy has changed considerably over time, and these days due to the variety of energy choices available and differing goals of their advocates, defining or treating some energy types as “alternative” is highly controversial.
However, in general when we refer to a souce as being an “Altenative” source, it is generally taken to mean that it is cleaner and more desirable than an existing source it will replace, ie.if there is an “alternative” energy source, then it presupposes that there is an existing undesirable source that it can replace.
No doubt as time and science progresses we will discover more and more alternative energy sources while some of the existing ones will be dropped or excluded.