Answer:
The Constitution is one source of law. When something is done in violation of the Constitution it is "unconstitutional". Legal vocabulary aside, that term means exactly what it says: contrary to the Constitution. Because the Constitution is a source of law, everything that is unconstitutional is also illegal.
Step-by-step explanation:
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title.