Externality

Externality: an effect whereby those not directly involved in taking a decision are affected by the actions of others.

Positive externality: exists when an individual or firm making a decision does not receive the full benefit of the decision.
http://economics.fundamentalfinance.com/positive-externality.php

Negative externality: occurs when an individual or firm making a decision does not have to pay the full cost of the decision.
http://economics.fundamentalfinance.com/negative-externality.php

Social cost= Private + External cost
MSC(Marginal Social Cost)= MPC(Marginal Private Cost)+ MEC(Marginal External Cost)
MSC=MPB(Marginal Private Benefit)= Social Optimum

External Benefit= good for you
External Cost= bad for you