Vasopressin: A relatively small (peptide) molecule that is released by the pituitary gland at the base of the brain after being made nearby (in the hypothalamus). Vasopressin has an antidiuretic action that prevents the production of dilute urine (and so is antidiuretic). A syndrome of inappropriate secretion of vasopressin which results in the inability to put out dilute urine, perturbs fluid (and electrolyte) balance, and causes nausea, vomiting, muscle cramps, confusion and convulsions may occur in association with oat-cell lung cancer, pancreatic cancer, prostate cancer, and Hodgkin's disease as well as a number of other disorders. Vasopressin has a vasopressor action and so can stimulate contraction of arteries and capillaries. Hence the name "vasopressin." Vasopressin is also known as antidiuretic hormone (ADH).