 Tobacco, one of the most widely used addictive substances in the world, is a plant native to the Americas and historically one of the half-dozen most important crops grown by American farmers.More specifically, tobacco refers to any of various plants of the genus "Nicotiana" (especially "N. tabacum") native to tropical America and widely cultivated for their leaves, which are dried and processed chiefly for smoking in pipes, cigarettes, and cigars; it is also cut to form chewing tobacco or ground to make snuff or dipping tobacco, as well as other less common preparations.From 1617 to 1793 tobacco was the most valuable staple export from the English American mainland colonies and the United States.