Source region: A large area of the earth's surface, where large masses of air originate with uniform temperature and humidity conditions characteristic of the region
In meteorology, an air mass is an air volume determined by its temperature and the amount of water vapour. Air masses span several hundreds or thousands of miles, and conform to the surface properties below them. They are categorized by latitude and by their areas of continental or maritime origin.
Still, from surface effects the air masses themselves are mild. The areas of the globe from which air masses are called source regions. A source area must have certain temperature and humidity properties which can stay constant for a considerable length of time to influence the air masses above it.