ATEX stands for Atmosphere Explosives

In ATEX areas the flammable liquids, vapours, gases or combustible dusts are likely to occur in quantities sufficient to cause a fire or explosion.

ATEX areas are also known as Hazardous Locations, Hazardous Areas or
Explosive Atmospheres.
From July 1st 2006 all the existing and new workplaces in EU must comply the ATEX regulations