As described by political scientist and constitutional scholar David Fellman:

Constitutionalism is descriptive of a complicated concept, deeply imbedded in historical experience, which subjects the officials who exercise governmental powers to the limitations of a higher law. Constitutionalism proclaims the desirability of the rule of law as opposed to rule by the arbitrary judgment or mere fiat of public officials …. Throughout the literature dealing with modern public law and the foundations of statecraft the central element of the concept of constitutionalism is that in political society government officials are not free to do anything they please in any manner they choose; they are bound to observe both the limitations on power and the procedures which are set out in the supreme, constitutional law of the community. It may therefore be said that the touchstone of constitutionalism is the concept of limited government under a higher law.

Wikipedia

Constitutionalism in Islam
In Islam, worldly government is bound by the Constitution as in the treaties with other peoples, like the Madina Charter, and furthermore our government as a religious community is bound by the Holy Quran. In the words of Hadhrat Mirza Bashir-ud-Din Mahmud Ahmad (ra):

The Ahmadiyya Head or Khalifa has no right to alter an ordinance contained in the Holy Book. The Khalifa is a deputy, not a dictator. A deputy it bound to authority in the same way as are all the others. (al-Fazl, April 5, 1949)

Absolute Democracy
Absolute democracy, or majoritarian dictatorship, is characterized by absolute, or unlimited, rule of the majority. Unchecked, overriding political power is in the hands of a simple majority (50.1 percent) of the adult citizens or their democratically elected representatives. (…) There are no legal or constitutional restraints on the authoritative decision making and action taking power of the majority.

Examples of absolute democracy in ancient history include the direct democracies that operated in Athens and some of the other city-states of ancient Greece. (…) In Athens, members of the minority were subject to trial, conviction, and punishment for dissent and other political crimes. Political leaders who had lost the support of the majority were subject to indictment and prosecution for “treason” and/or “deceiving the public” and could be easily and quickly convicted by a popular court, which was more like a nineteenth- century or early twentieth-century American local lynch mob than a contemporary American court of law…

An important example of absolute democracy occurring during the modern era of European history is the political regime that operated in France during that brief but most violent phase of the French Revolution known to history as the “Reign of Terror”, the period of Jacobin dictatorial rule from June 2, 1793, to June 27, 1794.

In the modern era, absolute democracies have been highly unstable and have had rather short lives. Generally, such political regimes have quickly collapsed, resulting in civil war or widespread lawlessness and violence, followed by autocracy or oligarchy ruthlessly imposed by military force and ruthlessly maintained by continuing resort to the usual brutal and downright barbarous methods of a tyrannical, thoroughgoing police-state.

The Relationship of Government and Civil Society in Constitutional Democracies and Authoritarian Regimes