What is scoliosis?

Scoliosis refers specifically to a condition characterized by an abnormal curvature of the spine, which can manifest as a lateral curvature that often resembles an "S" or "C" shape when viewed from behind. This diagnosis encompasses various forms and severities of spinal curvature.

While excessive outward curvature (option A) and excessive inward curvature (option B) describe specific types of spinal deformities, they do not encompass the broader definition of scoliosis itself. Option D relates to inflexibility of the back but does not provide a definition of scoliosis. In essence, the term "curvature of the spine" accurately captures the essence of scoliosis, making it the best choice.