Tobacco (Nicotiana spp., L.) refers to a genus of broad-leafed plants of the nightshade family indigenous to North and South America, or to the dried and cured leaves of such plants. Tobacco leaves are often smoked (see tobacco smoking) in the form of a cigar or cigarette, or in a smoking pipe, or in a water pipe or a hookah. Tobacco is also chewed, "dipped" (placed between the cheek and gum), and sniffed into the nose as finely powdered snuff. The Spanish word "tabaco" is thought to have its origin in Arawakan language, particularly, in the Taino language of the Caribbean, said to refer to a roll of these leaves (according to Bartolome de Las Casas, 1552) or to the "tabago", a kind of y-shaped pipe for sniffing tobacco smoke (according to Oviedo), but Sp. tabaco (also It. tobacco) was commonly used to define medicinal herbs from 1410, originating from the Arabic "tabbaq", reportedly since the 9th century, as the name of various herbs. The word might then be European, and later applied to this plant from the Americas.

Tobacco contains the alkaloid nicotine, a powerful neurotoxin that is particularly harmful to insects. All means of consuming tobacco result in the absorption of nicotine in varying amounts into the user's bloodstream, and over time the development of tolerance and dependence. Absorption quantity, frequency and speed seem to have a direct relationship with how strong a dependence and tolerance, if any, might be created. A lethal dose of nicotine is contained in as little as one half of a cigar or three cigarettes; however, only a fraction of the nicotine contained in these products is actually released into the smoke, and most clinically significant cases of nicotine poisoning are the result of concentrated forms of the compound used as insecticides. Other active alkaloids in tobacco include harmala alkaloids.

Tobacco smoking carries significant risks including the potential to develop various cancers as well as strokes, and severe cardiovascular and respiratory diseases. ^* Significantly shorter life expectancies have been associated with tobacco smoking. ^* Many jurisdictions have enacted smoking bans in an effort to minimize possible damage to public health caused by tobacco smoking. The substantially increased risk of developing cancer as a result of tobacco usage seems to be due to the plethora of nitrosamines and other carcinogenic compounds found in tobacco and its residue as a result of anaerobic heating, either due to smoking or to flue-curing or fire-curing. The use of flue-cured or fire-cured smokeless tobacco in lieu of smoked tobacco reduces the risk of respiratory cancers but still carries significant risk of oral cancer. ^* In contrast, use of steam-cured chewing tobacco (snus), avoids the carcinogenicity by not generating nitrosamines, but the negative effects of the nicotine on the cardiovascular system and pancreas are not ameliorated. ^*