A cost is SOMETHING THAT IS GIVEN UP WHEN A DECISION IS MADE.
For example, we talk about social costs to collect those consumption of factors (ex. natural or environmental), which are not borne by the manufacturers of a good and therefore excluded from the calculation of their economic costs, but on the contrary are paid by a whole community or by society as a whole. A typical case of social cost is the deterioration of the waters of a river derived from the installation of a factory of a certain good. If there is no environmental legislation, the manufacturing company will not take into account among its costs the environmental damages caused by the development of its activity and the damages derived from the decrease in the quality of the water would be borne by all the inhabitants of the area.