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.
How a Briton in Vienna chose the citizenship of his host country after the UK Referendum on EU Membership to negate the potential personal consequences of Brexit