What are the economic side effects or by-products that affect uninvolved third parties called?

The correct term for the economic side effects or by-products that affect uninvolved third parties is "externalities." Externalities occur when the actions of individuals or businesses have repercussions that impact others who are not directly involved in a transaction. These can be positive or negative. For example, negative externalities, such as pollution from a factory, impose costs on surrounding communities that are not reflected in the market price of the goods produced. Conversely, positive externalities, like the benefits of a well-maintained public park, provide advantages to people who didn't contribute to the park's upkeep.

Understanding externalities is crucial in economics because they highlight situations where market outcomes may not reflect true societal costs and benefits, leading to inefficiencies. This often calls for government intervention or policy adjustments to correct the market failure caused by externalities.