Angostura bitters, often simply referred to as angostura, is a concentrated bitters made of water, 45.6% alcohol, gentian root, and vegetable flavoring extracts by House of Angostura in Trinidad and Tobago. They are typically used for flavoring beverages, or (less often) food. The bitters were first produced in the town of Angostura (Ciudad Bolívar, Venezuela) (hence the name), and do not contain angostura bark. The bottle is easily recognisable by its distinctive oversized label.
Read more about Angostura Bitters: History, Medicinal Properties, Orange Bitters, Bitters Shortage of 2009, Cocktails With Angostura Bitters