A common-pool resource is a resource that is difficult to exclude people from using but whose use by one person reduces what remains available to others. Fisheries, aquifers, forests, grazing land, and some shared digital or institutional resources can have these characteristics.
Common-pool resources are not automatically doomed to collapse. Outcomes depend on boundaries, monitoring, trust, rules, enforcement, conflict-resolution mechanisms, and whether users can adapt governance to local conditions.
See the tragedy of the commons as a systems problem and principles for governing shared resources.