What is the definition or description of: syndrome of inappropriate antidiuretic hormone?

SIADH: Vasopressin (aka anti diuretic hormone) is a hormone that controls the free water content of the body through the regulation of water channels in the kidney. It is normally released in response to low blood pressure and highly concentrated blood. When ADH is released, urine concentrates and blood dilutes. In siadh, vasopressin is released in a atypical way due to pathological factors.