Some economic terms

Excludability

 Ecludability and non-exculdability:
In economics, a good or service is said to be excludable when it is possible to prevent people who have not paid for it from having access to it, and non-excludable when it is not possible to do so.

Rivalry

In economics, rivalry is a characteristic of a good. A good can be placed along a continuum ranging from rivalrous (rival) to non-rival. The same characteristic is sometimes referred to assubtractable or non-subtractable . A rival (subtractable) good is a good whose consumption by one consumer prevents simultaneous consumption by other consumers. Put differently, a good is considered non-rival (non-subtractable) if, for any level of production, the cost of providing it to a marginal (additional) individual is zero


Externality


Externality: 
In economics, an externality (or transaction spillover) is a cost or benefit, not transmitted through prices, incurred by a party who did not agree to the action causing the cost or benefit. A benefit in this case is called a positive externality or external benefit, while a cost is called a negative externality or external cost.




source: wikipedia