The physiological or pathological loss of substance. Bone resorption occurs due to the activity of osteoclasts; it allows tooth movement to take place during normal eruption, tooth migration, and orthodontic therapy. Horizontal bone resorption is a typical feature of chronic periodontitis in which the crestal margins of the alveolar bone are resorbed. Alveolar bone resorption around an individual tooth (vertical bone resorption) may be associated with occlusal traumatism and periodontal disease. Osteoclastic and cementoclastic activity can result in root resorption.