A constitutional government is the one that is defined by the constitution of the country. A constitution can merely be a set of fixed norms or principles or a legal instrument accepted in the country as a framework for the polity of the country.
Constitution of a country is supreme law of the land, it is empowered with the sovereign authority of the people (we the people) by the framers and the consent of the legislatures of the states, it is the source of all government powers, and also provides important limitations on the government that protect the fundamental rights of citizens.
It is a collection of fundamental principles or established precedents according to which a state or other organization is acknowledged to be governed. Its major impact is the limit that it puts on the sovereign governments on how to rule and govern, therefore a constitutional government is limited government.