While several grumbling automakers in India may have their own full form, BS actually stands for Bharat Stage - emission standards established by the Indian government to limit air pollutants coming out of internal combustion engines like those found in motor vehicles.

Bharat stage emission standards are emission standards instituted by the Government of India to regulate the output of air pollutants from internal combustion engines and Spark ignition engines equipment, including motor vehicles. The standards and the timeline for implementation are set by the Central Pollution Control Board under the Ministry of Environment & Forests and climate change.

The auto emission norms are emission standards which are adopted by the government of a nation to check the air pollutants released from any internal combustion engine equipment, including motor vehicles.  These norms were introduced in India in 2000, when the Bharat Stage norms were adopted by the then government, based on the European emission norms.

In around 2005 and 2006, BS III was rolled out in NCR and several other cities in a bid to check on pollution. This was almost similar to European standards (Euro III) which was in place between 2000 and 2005 in most western nations.

India had enforced Bharat stage III norms across the country since October 2010. In 13 major cities, Bharat stage IV emission norms were put in place since April 2010.

Source: firstpost.com, timesofindia.indiatimes.com, wikipedia.org