Public good
Overview
 
In economics
Economics
Economics is the social science that analyzes the production, distribution, and consumption of goods and services. The term economics comes from the Ancient Greek from + , hence "rules of the house"...

, a public good is a good that is non-rival
Rivalry (economics)
In economics, rivalry is a characteristic of a good. A good can be placed along a continuum ranging from rivalrous to non-rival. The same characteristic is sometimes referred to as subtractable or non-subtractable . A rival good is a good whose consumption by one consumer prevents simultaneous...

 and non-excludable. Non-rivalry means that consumption of the good by one individual does not reduce availability of the good for consumption by others; and non-excludability means that no one can be effectively excluded from using the good. In the real world, there may be no such thing as an absolutely non-rivaled and non-excludable good; but economists think that some goods approximate the concept closely enough for the analysis to be economically useful.

For example, if one individual visits a doctor there is one fewer doctor's visit for everyone else, and it is possible to exclude others from visiting the doctor.
 
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