Immigration refers to the movement of people between countries. While the movement of people has existe throughout human history at various levels, modern immigration implies long-term, legal, permanent residence. Short-term visitors and tourists are considered non-immigrants (see expatriate). Immigration across national borders in a way that violates the immigration laws of the destination country is termed illegal immigration. Under this definition, an illegal immigrant is a foreigner who either illegally crossed an international political border, be it by land, water, or air, or a foreigner who legally entered a country but nevertheless overstays his/her visa in order to live and/or work therein. Seasonal labour migration, while generally non-permanent in nature (typically for periods of less than a year), is often treated as a form of immigration.