Q. What do you mean by Externality?

An externality exists when the actions of one individual affect the wellbeing of other individuals without any compensation taking place. For illustration, the discharge of a CSO (combined sewer overflow) will be a negative externality to informal recreational users of a river corridor, to the extent that it will tends to an aesthetic degradation of the river, for which they will not receive any financial compensation. You may also have positive externalities, for example, an industry which uses a better waste treatment process will improve the river quality.