A Citizen as defined by the Oxford Dictionary is a person who has full rights in a country. A citizen could also be a person who is recognised by a country as being her citizen because such person possesses some chracteristics (e.g. To be born in that country) which entitles him or her to some rights in that country. Under the Nigerian 1999 constitution, one may be a citizen either by birth, registration or by naturalisation  ( see Section 25 – 32 of 1999 Constitution). 
Citizenship by birth.

 A citizen by birth in Nigeria is a person who was born in Nigeria before or after independence and whose parents or grandparents belonged to a community indigenous to Nigeria or any person born outside Nigeria either of whose parents is a citizen of Nigeria.
Citizenship by Registration. 
The President may register any person as a citizen of Nigeria if he is satisfied that such person is of good character; he has shown a clear intention to be domiciled in Nigeria and has taken the Oath of Allegiance. Such persons are usually women who are married to Nigerians or people born outside Nigeria who are of full age capacity and either of  whose grandparents are Nigerians.   
Citizenship by Naturalization.
 Any person may also apply to the President of Nigeria to be granted a certificate of naturalisation. However such person must satisfy the following conditions:
–  He must be a person of full age and capacity;
– He is a person of good character;
– He has shown a clear intention to be domiciled in Nigeria;
– He is in the opinion of the Governor where he plans to reside to be acceptable to the local community as has assimilated the way of life of Nigerians;
– He has made or is capable of making contributions to the advancement; progress and well – being of Nigeria;
– He has taken the Oath of Allegiance;
– He has resided in Nigeria for a period of fifteen (15) year;
– He has resided in Nigeria for a period of 12 months and preceding that has stayed in Nigeria for periods amounting in aggregate to not less than fifteen years.   
Persons who are citizens by registration or by naturalization may forfeit their Nigerian citizenship if he retains or acquires the citizenship of another country where he is not a citizen by birth and any person who is of full age capacity may renounce his Nigerian citizenship by declaration and the President shall cause the declaration to be registered. Such declaration will however not be accepted if made during any war in which Nigeria is physically involved or is contraty to public policy in the opinion of the President. 
Any citizen of Nigeria by registration or naturalization may be deprived of his citizenship if within 7 years after becoming a citizen such person is sentenced to imprisonment of not less than 3 years or if he has conducted himself in ways that shows disloyalty towards the Federal Republic of Nigeria. 
By: Adedunmade Onibokun Esq.