牙周炎又称牙周病 Periodontal disease，是牙周组织的疾病。侵犯牙龈、牙周、牙周膜、牙周韧带及牙槽骨等组织的慢性炎症。常见的慢性牙周病一般可分为牙龈炎和牙周炎，这两种临床症状不尽相同的疾病，一般笼统合称为牙周病。是一种破坏性疾病，由积聚在牙龈边缘的牙菌膜所分泌的毒素令牙周组织发炎所致。其主要特征为牙周袋的形成及袋壁的炎症、牙槽骨吸收和牙齿逐渐松动。它是导致成年人牙齿丧失的主要原因。

牙周病早期一般并无任何症状，可能会出现牙龈红肿及口臭。到了病情严重时，会出现牙齿松动、牙缝增阔、牙龈萎缩，到最后更可能整颗牙齿松脱，严重影响口腔咀嚼功能。

Periodontitis, also known as pyorrhea, is a set of inflammatory diseases affecting the periodontium, i.e., the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth. Periodontitis is caused by microorganisms that adhere to and grow on the tooth's surfaces, along with an over-aggressive immune response against these microorganisms. A diagnosis of periodontitis is established by inspecting the soft gum tissues around the teeth with a probe (i.e., a clinical examination) and by evaluating the patient's X-ray films (i.e., a radiographic examination), to determine the amount of bone loss around the teeth. Specialists in the treatment of periodontitis are periodontists; their field is known as "periodontology" or "periodontics".