What is a Hague Apostille?

A Hague Apostille (named after the Hague Convention) is attached to documents issued by Hague Convention signatories and serves the purpose of notarisation for that document. It is used, for example on copies of birth certificates for marrying abroad (although a sworn translation is required of it) and in the case of naturalisation is needed for a police record from the UK for submission to the Austrian authorities.