Common Resource
Plain definition
A common resource is non-excludable but rival: anyone can use it, but one person's use leaves less for everyone else.
In economics
Common resources suffer the tragedy of the commons — overuse — because users ignore the cost they impose on others. Solutions include regulation, tradable quotas, privatization (assigning property rights), or community norms as studied by Elinor Ostrom.
Example
Ocean fish stocks are a common resource: any boat can fish (hard to exclude), but each catch reduces the stock for other fishers.
Common confusion
A common resource is not the same as a public good; fish are rival (depletable) while national defense is not, so the policy problems differ.