Most of us know what it means for something to be unconstitutional. An unconstitutional act is one that violates some aspect of the Constitution as understood by the courts, although the public and its representatives are also free to make claims about the constitutionality of one act or another.
The courts have said, among other things, that racial segregation in public schools is unconstitutional; that unequal representation in state legislatures is unconstitutional; and that laws banning same-sex marriage are unconstitutional. Moving to the present, we can say that President Donald Trump’s order overturning birthright citizenship is, according to the plain text of the 14th Amendment, unconstitutional.
You get the picture.
But there are other ways to evaluate the actions of a government. You can ask a somewhat different question: not whether an action is constitutional, but whether it sits opposed to constitutionalism itself. You can ask, in other words, whether it is anti-constitutional.
Part of the conceptual basis of constitutionalism is a division between sovereignty and government. Sovereignty is the possession of supreme authority over the polity, and government is the instrument of that sovereignty. In an absolute monarchy or dictatorship, sovereignty belongs to the man or woman in charge, who commands the state in its entirety. In a constitutional system such as ours, sovereignty belongs to the people, who invest their authority in a set of rules and norms, a constitution.