Apostille
Certification of a U.S. document
The Apostille is a validation stamp ensuring that a particular document is recognized in foreign countries that are signatories of The Hague Convention of 5 October 1961—Abolishing the Requirement of Legalization for Foreign Public Documents.
Essentially, a document is only valid in the country in which it was issued. Validation for recognition in another country used to be a very complicated and time-consuming matter involving several authorities of the issuing country and validation by the consulate of the country in which the document was to be recognized.
In order to facilitate recognition of a document abroad, The Hague Convention, an international treaty establishing mutual recognition of documents, was enacted by many countries including the United States and Portugal.
According to this treaty a document originating from a signatory country is recognized by all other signatory countries if it bears the Apostille stamp, which is a validation performed by the superior office in the country and state where it was issued. If you have a document which needs an apostille, you should contact the office of the Secretary of State in the state where your document was issued.