Lordosis is historically defined as an abnormal inward curvature of the lumbar spine. However, the terms lordosis and lordotic are also used to refer to the normal inward curvature of the lumbar and cervical regions of the human spine. Similarly, kyphosis historically refers to abnormal convex curvature of the spine. The normal outward (convex) curvature in the thoracic and sacral regions is also termed kyphosis or kyphotic. The term comes from Greek lordos 'bent backward'.

Lordosis in the human spine makes it easier for humans to bring the bulk of their mass over the pelvis. This allows for a much more efficient walking gait than that of other primates, whose inflexible spines cause them to resort to an inefficient forward-leaning "bent-knee, bent-waist" gait. As such, lordosis in the human spine is considered one of the primary physiological adaptations of the human skeleton that allows for human gait to be as energetically efficient as it is.

Lumbar hyperlordosis is excessive extension of the lumbar region, and is commonly called hollow back or saddle back (after a similar condition that affects some horses). Sway back is a different condition with a different cause, that at a glance can mimic the outward appearance of lumbar hyperlordosis. Lumbar kyphosis is an abnormally straight (or in severe cases flexed) lumbar region.

Normal lordotic curvatures, also known as secondary curvatures, result in a difference in the thickness between the front and back parts of the intervertebral disc. Lordosis may also increase at puberty, sometimes not becoming evident until the early or mid-20s.^{[citation needed]}

In radiology, a lordotic view is an X-ray taken of a patient leaning backward.

Lumbar hyperlordosis is a condition that occurs when the lumbar region (lower back) experiences stress or extra weight and becomes arched more than normal, sometimes leading to muscle pain or spasms. It is a common postural position in which the natural curve of the lumbar region of the back is slightly or dramatically accentuated. Commonly known as swayback, it is common in dancers. Imbalances in muscle strength and length are one cause of this excessive stress to the lower back, such as weak hamstrings and tight hip flexors (psoai).^{[citation needed]} A major feature of lumbar hyperlordosis is a forward pelvic tilt, resulting in the pelvis resting on top of the thighs.

Other health conditions and disorders can cause hyperlordosis. Achondroplasia (a disorder where bones grow abnormally, which can result in short stature as in dwarfism), spondylolisthesis (a condition in which vertebrae slip forward), and osteoporosis (the most common bone disease in which bone density is lost resulting in bone weakness and increased likelihood of fracture) are some of the most common causes of hyperlordosis. Other causes include obesity, hyperkyphosis (spine curvature disorder in which the thoracic curvature is abnormally rounded), discitis (inflammation of the intervertebral disc space caused by infection), and benign juvenile lordosis. Other factors may also include rare diseases, including Ehlers–Danlos syndrome (EDS), where some joints throughout the body are so hyper-extensible that they can become unstable (i.e. problematically much more flexible than normal, frequently to the point of partial or full dislocation). With such hyper-extensibility, it is also quite common (if not the norm) for some of the muscles surrounding an unstable joint to compensate for that instability by contracting.

Excessive lordotic curvature – lumbar hyperlordosis, is also called "hollow back", and "saddle back" (after a similar condition that affects some horses); swayback usually refers to a nearly opposite postural misalignment that can initially look quite similar. Common causes of lumbar hyperlordosis include tight low back muscles, excessive visceral fat, and pregnancy. Rickets, a vitamin D deficiency in children, can cause lumbar hyperlordosis.