Electronic cigarette or electronic cigarette is a kind of hand-held electronic device, which can simulate the feeling of smoking. It works by heating the liquid to create a mist (often called "vapor") that the user inhales. The use of electronic cigarettes is often referred to as electronic cigarettes. The liquid in the electronic cigarette is called electronic liquid or electronic juice, which is usually made of nicotine, propylene glycol, glycerin and flavoring agent. Not all electronic liquids contain nicotine.

The health risks of e-cigarettes are uncertain. They may be safer than cigarettes, but they are less effective than other ways of quitting smoking. Their long-term health effects are unclear. They can help some smokers quit smoking. When non-smokers use electronic cigarettes, it will lead to nicotine addiction, so people worry that children may start smoking after using electronic cigarettes. So far, no serious adverse reactions have been reported in the trial. Less serious adverse reactions include throat and mouth irritation, vomiting, nausea, and coughing.

E-cigarettes produce an aerosol, commonly known as vapor, which is actually cut differently. Most of the toxic chemicals found in tobacco smoke do not exist in electronic smoke aerosol. Most of the smoke is less than 1% of the corresponding level of tobacco smoke. Aerosols can contain toxic substances and trace heavy metals at levels permitted by inhaled drugs, as well as potentially harmful chemicals not found in tobacco smoke at concentrations permitted by workplace safety standards. However, chemical concentrations may exceed more stringent public safety limits.

Modern electronic cigarettes were invented by Han lik, a Chinese pharmacist, in 2003. As of 2015, most electronic cigarettes were made in China. Since its first sale in 2004, its global usage has grown exponentially. In the United States and the United Kingdom, they are widely used. The reasons for using e-cigarettes include trying to quit smoking, reducing risk or saving money, although some people use them for entertainment. As of 2014, most users were still smoking. There are concerns that the dual use of tobacco products and electronic cigarettes could "delay or prevent smoking cessation". About 60% of users in the UK are smokers, while about 40% are former smokers. In the UK, the use of never smokers is negligible. Due to the overlap with tobacco laws and medical drug policies, many countries have debated the legislation of e-cigarettes. The 2016 European directive sets standards for liquids, evaporators, ingredients and child resistant liquid containers. As of August 2016, the FDA expanded its regulatory authority to include e-cigarettes. There are about 500 brands of e-cigarettes, with global sales of more than $7 billion.