The mass density of the universe which just stops the expansion of space, after infinite cosmic time has elapsed. The critical density is the boundary value between universe models that expand forever (open models) and those that recollapse (closed models). A measurement of the actual density of the universe could be compared to the critical density which would then, in principle, indicate the fate of the cosmos.
The critical density (with zero cosmological constant) is defined
The ratio of the actual density of the universe to the critical density is the cosmological parameter Omega. Closed Friedmann models have Omega greater than one, open Friedmann models have Omega less than one.