Why Oleum is known as fuming Sulphuric acid?

(1) Oleum can be represented by the formulaySO₃.H₂O where y is the total molar sulphur trioxide content .the value of y can be varied to different Oleum sample.
(2) Oleum also be expressed  as H₂SO₄.xSO3where x is molar free suphur trioxide.
(3) Oleum is the solution of of sulphur trioxide in sulphuric acid , it is also known as  fuming sulphuric acid or Pyrosulphuric acid(H₂S₂O₇=H₂SO₄+SO₃).
(4) Oleum sample contain two type of SO₃.

(i) Free SO3:- It is that SO₃ which combined with water water to give H₂SO₄ .

(ii) Combined SO3:- The SO₃ which is present in H₂SO₄ itself is known as combined SO₃ and it does not react with water.