Technically, it's volatile organic compounds and nitrogen oxides in the presence of sunlight, which result in the production of ozone.
Practically, it's dirty air caused by emissions from vehicles, agriculture and industry. And smog is making us sick.
Protective masks, air cleansing, smog apps – for many in Beijing, these are all part of everyday life. China's leadership has officially declared war on air pollution, but progress so far has been modest. The problem is complex and effective strategies need time. But people are tired of living under a blanket of smog.
The Indian capital New Delhi is sometimes referred to as the most polluted city in the world. Aletta André has been out and about in an auto rickshaw, finding out first-hand about the effects of air pollution at the time of year when it reaches peak levels.