What is the definition of critical density of the universe?

Question

What is the definition of critical density of the universe?

Answer 1

It's the density that would cause the Universe to stop expandind and start coming back together leading to a Big Crunch.  I think it has pretty much been disproven.

Answer 2

This is refering to the "big crunch" i think.

Basically, if the universe has enough density then it will eventually stop expanding and contract. Due to the gravitational attraction of all the mass.

If it is less dense then the gravitational attraction will not be enough to overcome the kinetic energy of the universe expanding. Thus the universe will forever expand. It will slow down, but never contract.

Basically it's the density which decides if the universe continues to expand, or stops... and contracts back together.