Dx is a medical abbreviation that usually means diagnosis. It may stand for donation instead of diagnosis, but that only applies in the context of organ transplants.
Dx stands for diagnosis in both human and veterinary medicine. It follows a common medical abbreviation style that uses the first letter of the word followed by the letter X. Other abbreviations include Sx for symptoms and Hx for history. The reason this style developed is unknown, but it may be based on the historical use of Rx for prescriptions. These abbreviations are usually used in medical histories or other less critical documents. They are avoided when writing prescriptions, patient instructions and care plans to prevent misunderstandings.