Broadly, the act of proving that something (e.g. a document) is true or genuine.
([Hague Convention Abolishing the Requirement of Legalisation for Foreign Public Documents, 5.10.1961](https://assets.hcch.net/docs/b12ad529-5f75-411b-b523-8eebe86613c0.pdf))
[...] legalisation means only the formality by which the diplomatic or consular agents of the country in which the document has to be produced certify the authenticity of the signature, the capacity in which the person signing the document has acted and, where appropriate, the identity of the seal or stamp which it bears.
‘Legalisation’ is a legal term of art that specifically refers to the process laid down in the Hague Apostille Convention. ‘Legalisation’ and ‘legalise’ should therefore be reserved for situations involving the Hague Apostille Convention process. More commonly, ‘legalise’ means to make something that was previously illegal permissible by law, which has nothing to do with authentication.
Based on Black’s Law Dictionary, West Group, 1999, p. 127 (6.3.2019).