What is an antidiuretic hormone and how does it help regulate an abnormal increase in solute?

Antidiuretic hormone (ADH): 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). ADH has an antidiuretic action that prevents the production of dilute urine (and so is antidiuretic). ADH is also known as vasopressin.

As with most processes in the body, it works in what is called a negative feedback situation.

This feedback system reduces the output of a system. (In physiology) a decrease in function in response to a stimulus. For example, the secretion of follicle-stimulating hormone decreases as the amount of circulating estrogen increases.