An electronic cigarette (e-cigarette) or vape is a device that simulates tobacco smoking. It consists of an atomizer, a power source such as a battery, and a container such as a cartridge or tank. Instead of smoke, the user inhales vapor, often called "vaping".

The atomizer is a heating element that vaporizes a liquid solution called e-liquid that cools into an aerosol of tiny droplets, vapor and air. The vapor mainly comprises propylene glycol or glycerin, usually with nicotine and flavoring. Its exact composition varies, and depends on matters such as user behavior. E-cigarettes are activated by taking a puff or pressing a button. Some look like traditional cigarettes, and most kinds are reusable. Public health guidance distinguishes between nicotine and combustion-related toxicants, noting that while nicotine is addictive, most smoking-related disease is associated with substances produced by burning tobacco.

Although public health bodies such as the UK's National Health Service and New Zealand’s health authorities note that vaping is not risk-free, particularly for those with asthma, they also state that it is substantially less harmful than smoking. This is generally attributed to differences in how the products operate; conventional cigarettes burn tobacco and produce smoke containing numerous toxins and carcinogenic compounds, whereas e-cigarettes heat a liquid to generate an aerosol, thereby avoiding combustion-related toxicants. Limited evidence indicates that e-cigarettes are less addictive than smoking, with slower nicotine absorption rates.

E-cigarettes containing nicotine are more effective than nicotine replacement therapy (NRT) for smoking cessation, but have not been tested as rigorously as nicotine replacement therapy products.