The reader's digest version answer: "homogenized tobacco product" aka "sheet tobacco".
"What it's not a mix of assorted fix premium tobaccos like it says on the package ?" as some geniuses like to ask.
No Virginia it ain't ! It's a combination of:
* Blended leaf (BL) sheet is a thin, dry sheet cast from a paste made with tobacco.
* Expanded (ES) are rolled, flattened, and shredded leaf stems that are expanded by being soaked in water and rapidly heated. Improved stems (IS) follow the same process but are simply steamed after shredding.
* Reconstituted leaf (RL) sheet: a cardboard plywood-like material made from recycled tobacco , tobacco stems and "class tobacco", that are collected at any stage of tobacco processing. RL is made by extracting the soluble chemicals in the tobacco by-products, processing the leftover tobacco fibres from the extraction into a paper.
A combination of sheet tobacco will be mixed with humectants such as propylene glycol or glycerol, as well as flavoring products and enhancers such as cocoa, licorice, tobacco extracts, and various sugars...
The sheet will then be shredded, along with a specified amount of small laminate, expanded tobacco, BL, RL, ES and IS. A chemical(s) flavor/fragrance, called the "topping" , will then be blended into the tobacco mixture, which replaces lost flavors due to the repeated wetting and drying used in processing the tobacco.
Actually, it's probably the burnt carmel sugars that cause much of the addiction associated with cigarette depravity. And the added ammonia and anti-freeze chemicals that amplify the health hazards of ingesting particulate dust matter.
So the next time you light up a cigarette remember that it's almost the same stuff they use to use for siding those McMansions...