Epstein-Barr virus, or EBV, is one of the most common human viruses in the world. Read medical definition of Epstein-Barr virus. Epstein-Barr is the virus that causes mononucleosis. Epstein-Barr virus (EBV), also known as human herpesvirus 4, is a member of the herpes virus family. A common virus that remains dormant in most people.