What is the term for a system of government where the local and regional governments derive all authority from a strong national government?
The term for this sort of a system of government is “unitary system.” A unitary system is a system in which there are no subdivisions of government that have powers to which they are entitled by right. The entire existence of any lower levels of government occurs only if the national government wishes. This system of government is typically used in countries that are small or that are rather homogeneous. For example, Japan has such a system.
This system is typically discussed along with two other systems. The opposite of a unitary system is a confederal system. In this system, the national government only has whatever powers the state governments give it. The United States had such a system under the Articles of Confederation. Somewhere between a unitary system and a confederal system is a federal system. This is what the US has today. It is a system in which both the national government and the state governments are assigned powers by a constitution.