From Statistics Explained
Citizenship is the particular legal bond between an individual and his or her State, acquired by birth or naturalisation, either by declaration, choice, marriage or other means under national legislation.
Third country nationals is any person who is not a citizen of the European Union (EU), including stateless persons - see Art. 2.1 (i) of the Council Regulation (EC) no 862/2007.
A stateless person is not considered as a national by any State under the operation of its law. This includes also a person whose nationality is not established.
A recognised non-citizen is a person who is not a citizen of the reporting country nor of any other country, but who has established links to that country which include some but not all rights and obligations of full citizenship. Recognised non-citizens are not included in the number of European Union (EU) citizens.