Nicotine dependence is similar to other forms of drug dependence, and refers to the need of smokers to have nicotine to function properly.
The level of nicotine dependence is a combination of many different factors that can vary between individuals. Dependence on nicotine (and any other addictive drug) is made up of psychological dependence, physical dependence and tolerance. This means that people become dependent (or ‘addicted’) to the psychological effects of nicotine (e.g. stress relief), and the body becomes physically dependent on nicotine, such that without it the body has withdrawal symptoms.
Tolerance is the process whereby smokers become used to nicotine. As they continue smoking, they need more and more nicotine to achieve the same psychological and physical effect.
For more information, see Smoking.