state governed as a single unit with a supreme central government
A unitary state is a state whose three organs of state are ruled constitutionally as one unit, with central legislature. It differs from a federal state, in which the authority is divided between the head (for example the central government of a country) and the political units governed by it (for example the municipalities or provinces of the country), and also granting some degree of autonomy to those political subdivisions (e.g. As the permission to create their own regional laws, for example).
These are examples of unitary states: