Legitimatise is a term used to describe the process of making something lawful or official. However, there are several other synonyms that can be used to express similar meanings. For instance, the word 'legalize' means to make something lawful or to authorize it officially by law. 'Authorize' means giving permission or approval to something. 'Validate' means to declare as genuine, reliable or acceptable by determining its truth and accuracy. 'Sanction' means to give official permission or approval, thus making it acceptable or legitimate. 'Approve' means to accept or give assent to something, making it valid or permissible. Overall, these synonyms serve to express the same meaning as legitimatise.