Answer:
when individuals take external costs and benefits into account in their decision making.
Step-by-step explanation:
In economics, an externality is defined as the benefit or cost that has an impact on someone or a third party who had no intention of incurring the cost or the benefit. Externalities are either positive or negative. However, internalizing an externality simply implies moving away the costs or burden from a negative externality from outside to inside. Taking external costs and benefits and putting them into account in a decision making is simply an example of internalizing an externality.