Final answer:
The noise from the new airport runway that negatively affects neighbors is a negative externality, which occurs when a third party is impacted by an economic activity without their consent.
Step-by-step explanation:
The noise from the new airport runway that is bothering the neighbors is an example of a negative externality. This type of externality occurs when an economic activity causes external costs or benefits to third-party stakeholders who did not choose to incur that cost or benefit. In this case, the increase in noise pollution negatively affects the neighbors' quality of life even though they are not directly involved in the airport's operations.
Externalities can be either positive or negative. A positive externality would be a benefit that is enjoyed by a third-party; for example, if you enjoy bird watching and your neighbor's yard attracts birds, you benefit without any effort on your part. Conversely, a negative externality like noise from airplanes results in an unaccounted-for cost to others. Pollution is a classic example of a negative externality. In the provided example, the neighbors did not partake in the economic activity of expanding the airport, yet they suffer from the additional noise.