Oleum is the term used to describe fuming sulfuric acids.
Oleum is made in various strengths and consists of SO3 dissolved in 100% H2SO4. Thus, for example, 20% oleum contains 20% SO3 and 80% H2SO4 by weight.
Oleum is normally turbid and off-white in appearance. Depending on strength, the vapor pressure is such that SO3 fumes escape and combine with moisture in the air to form sulfuric acid mist particles that are visible and can create clouds of dense, white fumes.