A crater lake is a lake which forms in a volcanic caldera after the volcano has been inactive for some time. The most well known crater lake, that also bears the same name as the geological feature, is Crater Lake in Oregon state's Mount Mazama. Some crater lakes are acidic, however some remain fresh water. Some may also have geothermal activity, especially if the volcano is still active, but merely dormant.
Some crater lakes also form in the craters left by impact events. These do not tend to develop acidity or geothermal activity, lacking ongoing volcanic processes.