Smoking refers to the inhalation of smoke produced by burning tobacco (most commonly in cigarettes, cigars, pipes, or hookahs), which delivers nicotine—a highly addictive stimulant alkaloid—along with thousands of chemicals, including tar, carbon monoxide, formaldehyde, benzene, and numerous carcinogens, directly into the lungs and bloodstream. It is the leading preventable cause of death worldwide, strongly linked to lung cancer, chronic obstructive pulmonary disease (COPD/emphysema/chronic bronchitis), heart disease, stroke, peripheral artery disease, various other cancers (mouth, throat, esophagus, pancreas, bladder, cervix, kidney), reduced fertility, pregnancy complications (low birth weight, preterm delivery, sudden infant death syndrome), weakened immune function, accelerated skin aging (wrinkles, yellowing), gum disease/tooth loss, cataracts, type 2 diabetes, and rheumatoid arthritis. Secondhand smoke exposure also increases risks of lung cancer, heart disease, asthma attacks, and sudden infant death in nonsmokers, particularly children. The habit typically begins with social or experimental use, progresses due to nicotine dependence causing cravings, withdrawal symptoms (irritability, anxiety, difficulty concentrating, increased appetite), and reinforced pleasure or stress relief, often becoming a chronic, compulsive behavior despite awareness of harms.