An externality (a type of market failure) exists where a market price omits environmental costs and/or benefits. In such a situation, rational (self-interested) economic decisions can lead to environmental harm, as well as to economic distortions and inefficiencies.
Environmental pricing reform can be economy-wide, or more focussed (e.g. specific to a sector or environmental issue such as climate change. A "market based instrument" or "economic instrument for environmental protection" is an individual instances of Environmental Pricing Reform. Examples include green tax-shifting (ecotaxation), tradeable pollution permits, or the creation of markets for ecological services.