Perspectives are different frameworks from which to explore the knowledge around sustainable sanitation and water management. Perspectives are like filters: they compile and structure the information that relate to a given focus theme, region or context. This allows you to quickly navigate to the content of your particular interest while promoting the holistic understanding of sustainable sanitation and water management.
A negative externality is a term used in economic theory. It applies, when the cost a consumer pays is smaller than the social costs of the product. The external costs are then passed on to society. A common example for a negative externality is pollution.