Lordosis is the excessive increase of the normal lumbar curvature of the vertebral column.
Also called hollow back, saddle back, and swayback.
Lumbar and cervical vertebras have lordotic curvatures, also known as secondary curvatures (which are concave towards the back). Secondary curvatures of the vertebral column are caused mainly because of the difference of thickness between the anterior and posterior part of the intervertebral disc. Those curvatures start to appear during the fetal period, but don't become evident until lactation.
The familiar concave shape of the horse's back is, by extension, described as lordotic.