What is the definition or description of: anti-diuretic hormone?

Vasopressin: Anti-diuretic hormone, also known as vasopressin, is a chemical compound which signals to the kidney that it should re-absorb water which would otherwise be urinated. Too much or too little antidiuretic hormone can cause abnormalities of the sodium level in the blood.

ADH (vasopressin): ADH (vasopressin) is a hormone designed to help the kidneys retain water in volume contracted states.

Vasopressin: ADH (vasopressin) is a hormone secreted by the posterior pituitary which has various influences on the body , one of them being that it effects the collecting tubule of the filtration system in the kidney preserving free water for the body- so concentrates the urine( less free water out).