From Envirowiki

Clean energy refers to a number of energy sources that do not produce toxins or pollutants. Most frequently, it refers to electricity production that does not emit greenhouse gasses.

The only currently available clean energy is renewable energy, such as wind or solar power, although there are a number of other clean energy sources could exist, such as hydrogen power, which would mainly emit water, or nuclear fusion which would emit helium.

[edit] UnClean Energy

Clean coal, an oxymoron and artwork of propaganda by the coal lobby, is often conflated with clean energy, but coal can never be "clean". Clean coal simply refers to coal burning that stored the by-products (chiefly carbon dioxide) are stored somewhere. This doesn't actually deal with the by-products, it just leaves them to be dealt with by future generations. Nuclear power is similar - the only major difference is that it's by products are radioactive for hundreds of thousands of years, causing cancer and other problems, instead of climate change.