Hyperlordosis is an excessive inward curve of the lower back that tips the pelvis forward and produces the arched, hollow-backed posture often called swayback. It is also what people are describing when they ask why their back curves inward, why their lower back dips in, or why their spine looks sunken or indented. Seen from the side, a healthy spine curves inward at the neck and the lower back, a shape called lordosis, and outward in the middle back, a shape called kyphosis. An inward curve in the lower back is normal and necessary, however hyperlordosis occurs when that curve becomes too deep. The two words are often confused: lordosis is the healthy curve everyone has, while hyperlordosis is an exaggerated version of it.
Hyperlordosis can have different underlying causes. Sometimes it is simply posture and muscle balance, and sometimes the lower back is curving more to compensate for a problem elsewhere, such as a slipped vertebrae. What matters is why the curve is there, and whether it is flexible or fixed.