A closed universe is a cosmological model characterized by a positive curvature, similar to the surface of a sphere. In this model, the total density of matter exceeds the critical density, meaning there is enough gravitational attraction to eventually stop the universe's expansion and cause it to eventually reverse. 

 In a closed universe, the geometry differs from Euclidean geometry, meaning parallel lines can eventually meet, and the angles of a triangle will sum to more than 180 degrees. The ultimate fate of a closed universe is often described as a "Big Crunch," where all matter collapses back into a singularity, suggesting a cyclical nature of the universe's existence.