Cambodia is a country in Southeast Asia that borders the Gulf of Thailand. Neighboring countries include Laos, Thailand, and Vietnam. The geography of Cambodia is mountainous in the southwest and north and is dominated by the Mekong River and Tonle Sap Lake. The government system is a multiparty democracy under a constitutional monarchy; the chief of state is the king, and the head of government is the prime minister. Cambodia has a mixed economic system, self-defined as a planned economy with markets, in which the economy includes a variety of private freedom, combined with centralized economic planning and government regulation. Cambodia is a member of the Association of Southeast Asian Nations (ASEAN).