oleum
A solution of sulfur trioxide in sulfuric acid.
[U] Uncountable
This term describes a highly corrosive, aggressive chemical agent that behaves as a superacid. It is characterized by its extreme reactivity and the violent heat released when it comes into contact with water.
In industrial settings, it serves as a critical intermediate for creating various sulfonated products. It is treated with far more caution than standard sulfuric acid due to its volatility and the immediate danger it poses to skin and respiratory systems.
Used as a mass noun for the chemical substance, such as saying there is a liter of oleum in the flask.