Any currency that is used as an alternative to the dominant national currency. This could be a foreign currency, a locally created currency, or an crypto currency such as Bitcoin. It can also include commodities used as currency (salt, in Ethiopia), barter, service hours, etc. Virtually anything that is traded for value can legally become a currency, if people agree to it.
Crypto currencies are not readily sustainable because of cost, and pollution. A currency that uses energy intensively when fossil fuels remain the dominant energy source and have been demonstrated to heavily and adversely impact health, ecological systems and even the climate is a design that can be improved upon. Currently, there is a massive movement to stop using fossil fuels, yet renewable energy cannot be produced fast enough to replace them because of limited physical resources and capacity among other challenges.
Adding yet more energy needs under that circumstance, when many developing countries are already doing that creates a problem that must be weighed along with any solution the coin offers. Bitcoin and other cryptocurrencies tend to be risky & highly speculative investments as opposed to relatively stable currencies. Permaculture, whole systems and ecological designers are endeavoring to design alternative currencies that eliminate or greatly reduce the energy issue and others. Two examples are SEEDS (https://joinseeds.earth/) and Regen Network (https://www.regen.network/).