Constitutional law is the body of law that establishes, distributes, limits and regulates the exercise of public power in a State — the law of the constitution. It answers four questions every organised State must answer: who governs (the organs — legislature, executive, judiciary); how they are chosen and removed; what each may do (the allocation and limitation of power); and what the individual may demand against the State (fundamental rights and remedies).
The word "constitution" works in three ascending senses — constitutional law tracks all three: The widest (systemic) sense: the constitution of a State is its whole system of governance — every rule, written and unwritten, legal and non-legal, that regulates the government. In this sense every State has a constitution, because no State can function without some rules allocating power. The lawyer's (documentary) sense: the Constitution is a single, solemn, written instrument — usually adopted by a special constituent process and entrenched against easy change — that constitutes the organs of the State and limits them in favour of rights. The Constitution of Nepal (2015) is a constitution in this sense. The constitutive sense: a constitution constitutes — it creates what did not exist before it: the President, the Federal Parliament, the Supreme Court, the seven provinces. It is the legal fountainhead of all public authority: every act of government traces its legality back to it.
Classic definitions to quote: Aristotle: the constitution is "the arrangement of offices in a State" — the way power is organised. Lord Bolingbroke: the constitution is "that assemblage of laws, institutions and customs... which compose the general system, according to which the community hath agreed to be governed." K.C. Wheare: the constitution is "the whole system of government of a country, the collection of rules which establish and regulate or govern the government."
Carl J. Friedrich: the constitution is "the process by which governmental action is effectively restrained" — limitation, not mere organisation. Working definition for this course: constitutional law is the fundamental law that constitutes the organs of the State, distributes power among them, limits that power in favour of the people's rights, and stands supreme over all ordinary law.
Dicey's two-part split (learn this — it structures 1.6 and 1.8): A.V. Dicey divided English constitutional law into (i) "constitutional law strictly so called" — enforceable rules a court will apply; and (ii) constitutional conventions — rules of political morality enforced not by courts but by public opinion and Parliament. Constitutional law is thus both narrower and wider than the constitutional document: narrower, because parts of the document are political programme (directive principles); wider, because much of living constitutional law (conventions, interpretation, statutes) lives outside the document.