A good is nonexcludable if it is impossible to exclude non-paying individuals from consumption of the good. Eg. streetlight consumption cannot be excluded from people living in the street who did not pay for its installation.

A good is rival if its consumption by one consumer prevents simultaneous consumption by other consumers. Eg. a jacket can only be worn by one person at a time

If a good is both nonexcludable and rival in consumption, then it is common good (Eg. open areas for grazing)