Ω0 = ρ / ρc
where (ρ ) is the actual density of the Universe and (ρc ) is the critical density (the average density of matter required for the Universe to just halt its expansion, but only after an infinite time).
This relation determines the overall geometry of the universe.
When the ratio is exactly equal to 1, the geometry of the universe is flat (Euclidean) and contains enough matter to halt the expansion but not enough to re-collapse it.
If Ω0 is less than 1, the Universe is open and will continue to expand forever. If Ω0 is greater than 1, the Universe is closed and will halt its expansion and re-collapse.
The current critical density is given by: