Why is non-rivalry characteristics considered a public good?

Non-rivalry is considered a characteristic of public goods because one person's consumption of the good does not diminish its availability for others. This means that multiple individuals can benefit from the good simultaneously without reducing its value or access for anyone else. Examples include clean air or national defense, where everyone can enjoy the benefits without interfering with others' consumption. As a result, non-rivalrous goods are often underprovided in a free market, necessitating government intervention or collective action to ensure their availability.