A flat universe is a model in cosmology where the overall geometry of the universe is flat, meaning it has zero curvature. In this scenario, the total density of matter is exactly equal to the critical density needed to halt its expansion but only after an infinite time. 

This results in a universe that will continue to expand forever but at a rate that slows down over time, approaching but never reaching a complete stop. In a flat universe, the laws of Euclidean geometry apply on a cosmic scale, which means that parallel lines will never meet and the angles of a triangle will add up to 180 degrees.