What is the addictive substance found in tobacco and why is it considered a stimulant?

The addictive substance in cigarettes is Nicotine. Unusually, Nicotine is both a stimulant, and a depressant, which is one of the reasons it is so addictive both physically and mentally. It excites and clears your mind like caffeine (stimulant) , but at the same time calms and relaxes you, like alcohol (depressant).