ELI5: Explain Like I'm 5

Externality

An externality is something that affects someone who isn't directly involved in an activity. For example, when one person smokes, it can cause problems for other people who have to breathe the smoke or even just smell it nearby. That's an example of an externality: the people who are affected don't do anything to cause it, they just happen to be in the wrong place at the wrong time.