A person who lives in an region undergoing a change of sovereignty and thus may choose between retaining their old citizenship or opting for the citizenship of the new sovereignty.
1914,W. R. Prior,Oxford Pamphlets: North Sleswick under Prussian Rule, London: Oxford University Press, page 9:
According to one authority, nearly 40,000 of the Sleswick Danes had become 'optants'—that is, had taken the 'option' of Danish nationality—or had emigrated, by the end of 1880.