A commons is a piece of land that belongs to everyone in a community. The commons in the middle of a village might be a green space that's available for gatherings and celebrations.
One way to think of the commons is as public land or space — roadways, town parks, public beaches, and sidewalks could all potentially be defined as part of the commons. Nobody owns the commons, and when some part of it becomes privately owned, it's called privatization. The word commons comes from the Latin root communis, which means both "in common, shared by all, or general," and also "common property."