Definitions for "Critical density"
The density of the Universe necessary so the expansion rate of the Universe is just barely sufficient to prevent a recollapse. Omega is the mass density of the Universe normalised to units of the critical density.
The minimum density needed for gravity to overcome the expansion of the universe and cause it to eventually re-collapse.
the density of the Universe necessary so the expansion rate of the Universe is just barely sufficient to prevent a recollapse. Numerically the critical density is 3*Ho2/[8*pi*G] or 19*[Ho/100]2*10-30 gm/cc.