What is Social Cost?

Legal Definition
Social cost in economics may be distinguished from "private cost". Economic theorists model individual decision-making as measurement of costs and benefits. Social cost is also considered to be the private cost plus externalities. Rational choice theory often assumes that individuals consider only the costs they themselves bear when making decisions, not the costs that may be borne by others.

With pure private costs, the costs carried by the individuals involved are the only economically meaningful costs. The choice to purchase a glass of lemonade at a lemonade stand has little consequence for anyone other than the seller or the buyer. The costs involved in this economic activity are the costs of the lemons and the sugar and the water that are ingredients to the lemonade, the opportunity cost of the labour to combine them into lemonade, as well as any transaction costs, such as walking to the stand.
-- Wikipedia