Spherical Aberration
A softening caused by rays passing through the outer zones of a spherical lens surface focusing nearer than those through the centre.
In depth
The result is a sharp core surrounded by a soft halo, which lowers contrast without entirely destroying detail. Stopping down blocks the offending outer rays and cleans the image up quickly. Some portrait lenses leave a controlled amount of it in deliberately, because the glow flatters skin, and a few offer a dial to vary it.
Examples in practice
A classic 85 mm portrait lens shot at f/1.8 renders skin with a soft luminous bloom that disappears by f/4.