Passive smoking is the inhalation of tobacco smoke, called passive smoke, secondhand smoke (SHS) or environmental tobacco smoke (ETS), by individuals other than the active smoker. It occurs when tobacco smoke diffuses into the surrounding atmosphere as an aerosol pollutant, which leads to its inhalation by nearby bystanders within the same environment. Exposure to secondhand tobacco smoke causes many of the same health effects caused by active smoking, although at a lower prevalence due to the reduced concentration of smoke that enters the airway.

According to a World Health Organization (WHO) report published in 2023, more than 1.3 million deaths are attributed to passive smoking worldwide every year. The health risks of secondhand smoke are a matter of scientific consensus, and have been a major motivation for smoking bans in workplaces and indoor venues, including restaurants, bars and night clubs, as well as some open public spaces.

Concerns around secondhand smoke have played a central role in the debate over the harms and regulation of tobacco products. Since the early 1970s, the tobacco industry has viewed public concern over secondhand smoke as a serious threat to its business interests. Despite the industry's awareness of the harms of secondhand smoke as early as the 1980s, the tobacco industry coordinated a scientific controversy with the purpose of stopping regulation of their products.

Fritz Lickint created the term "passive smoking" ("Passivrauchen") in a publication in the German language during the 1930s. Terms used include "environmental tobacco smoke" to refer to the airborne matter, while "involuntary smoking" and "passive smoking" refer to exposure to secondhand smoke. The term "environmental tobacco smoke" can be traced back to a 1974 industry-sponsored meeting held in Bermuda, while the term "passive smoking" was first used in the title of a scientific paper in 1970. The Surgeon General of the United States prefers to use the phrase "secondhand smoke" rather than "environmental tobacco smoke", stating that "The descriptor 'secondhand' captures the involuntary nature of the exposure, while 'environmental' does not." Most researchers consider the term "passive smoking" to be synonymous with "secondhand smoke". In contrast, a 2011 commentary in Environmental Health Perspectives argued that research into "thirdhand smoke" renders it inappropriate to refer to passive smoking with the term "secondhand smoke", which the authors stated constitutes a pars pro toto.

The term "sidestream smoke" is sometimes used to refer to smoke that goes into the air directly from a burning cigarette, cigar, or pipe, while "mainstream smoke" refers to smoke that a smoker exhales.

Secondhand smoke causes many of the same diseases as direct smoking, including cardiovascular diseases, lung cancer, and respiratory diseases. These include:

Epidemiological studies show that non-smokers exposed to secondhand smoke are at risk for many of the health problems associated with direct smoking.^{[citation needed]}

In 1992, a review estimated that secondhand smoke exposure was responsible for 35,000 to 40,000 deaths per year in the United States in the early 1980s. The absolute risk increase of heart disease due to ETS was 2.2%, while the attributable risk percent was 23%. A 1997 meta-analysis found that secondhand smoke exposure increased the risk of heart disease by a quarter, and two 1999 meta-analyses reached similar conclusions.