What term describes the theoretical density required for a flat universe?

The correct term for the theoretical density required for a flat universe is critical density. Critical density is the specific density value that separates the different geometrical configurations of the universe. In cosmology, it determines whether the universe will continue to expand forever or eventually collapse.

A flat universe occurs when the actual density of the universe equals the critical density. This results in a geometry where parallel lines remain parallel, and the universe has a Euclidean structure on large scales. It indicates that the gravitational force of the universe's mass is perfectly balanced with the rate of its expansion, leading to a universe that is stable in its expansion rate over time.

In contrast, other terms like maximum density, static density, and universal density do not have the same defined meanings in cosmological contexts and do not specifically represent the condition for a flat universe. Hence, critical density is the precise term used to describe this theoretical measurement in cosmology.