What is meant by density in units of critical density?

[indie452]

what is meant by density, such as the baryonic matter density in cosmology, in "units of the critical density"?

its mentioned in a question i need to do but i don't really know what it means.

 

[sgd37]

that means number of baryons per unit volume of space. The critical density is the density of matter in the universe such that the universe would be flat and it's 9.9 x 10-30 g/cm3