Public good
In 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.