Answer:
What is the definition of an externality?
What are externalities, and how do they affect markets?
Explanation:
There are positive and negative externalities.
Positive externalities occur when an economic transaction positively affects the well being of other people (or beings) that were not part of the transaction, e.g. a company installs solar panels to reduce the consumption of electricity, this action benefits not only the company but the whole environment.
Negative externalities occur when an economic transaction negatively affects the well being of other people (or beings) that were not part of the transaction, e.g. a company pollutes the air and the whole community suffers from the bad quality of the air.