Hepatitis D, also known as delta hepatitis, is a viral infection that affects the liver. It is caused by the hepatitis delta virus (HDV), a defective virus that requires the presence of the hepatitis B virus (HBV) to replicate. Hepatitis D is considered one of the most severe forms of viral hepatitis, as it can cause more severe acute and chronic hepatitis than HBV alone. Along with delta hepatitis, other synonyms for hepatitis D include hepatitis delta virus infection, HDV infection, and coinfection with HBV and HDV. While it is a rare form of hepatitis, it is important for individuals with chronic HBV infection to be aware of the potential risk of hepatitis D.