The average density of the universe needed to make its curvature flat.
In cosmology, the density that marks the transition from an open to a closed universe; the density that provides enough gravity to just bring the expansion to a stop after infinite time.
An average density (of the universe) that if exceeded will cause an eventual collapse of the universe. Alexander Friedmann calculated that figure to be about 2 x 10 -29 grams/cubic centimeter. An average density of that value or less means that the universe will continue to expand.