Ethnicity, Religion, Language, Territory, History and/or Culture are the features in which a 'nation' defines itself. Generally, particular uniformity of these aspects are what nationalist ideals are centered around. The term nation itself is heavily debated on origin and popular usages are more than muddled from its lingual history.
I think in layman terms most would assume the concept of nations as referring to the geographical areas, history and a culture that developed over time from those aspects that eventually reached a semi-homogeneous bureaucratic functionality.