Orthotics (Greek: Ορθός, romanized: ortho, lit. 'to straighten, to align') is a medical specialty that focuses on the design and application of orthoses. An orthosis (plural: orthoses) is "an externally applied device used to modify the structural and functional characteristics of the neuromuscular and skeletal system". An orthotist is the primary medical clinician responsible for the prescription, manufacture and management of orthoses.
An orthosis may be used to:

Control, guide, limit and/or immobilize an extremity, joint or body segment for a particular reason
Restrict movement in a given direction
Assist movement generally
Reduce weight bearing forces for a particular purpose
Aid rehabilitation from fractures after the removal of a cast
Otherwise correct the shape and/or function of the body, to provide easier movement capability or reduce painOrthotics combines knowledge of anatomy and physiology, pathophysiology, biomechanics and engineering. Patients who benefit from an orthosis may have a condition such as spina bifida or cerebral palsy, or have experienced a spinal cord injury or stroke. Equally, orthoses are sometimes used prophylactically or to optimise performance in sport.