Definition of niger: The Republic of the Niger, is a landlock country in west africa named after the Niger river. Niger covers a land area of approximately 1,270,000 km² (490,000 sq mi), making it the largest country in west Africa. Over 80% of its land area lies in the Sahara desert. The country’s predominantly Muslim population of about 22 million live mostly in clusters in the far south and west of the country. The capital and largest city is Niamey, located in Niger’s southwest corner.