a) an area of land in the United Kingdom or Ireland that is open to the public at all times and until the 18th century would have been land that was free for anybody to graze their animals on (often shortened to common).
The common land up at Chaileys always good for a walk.
b) collectively, all the common land in one of those two countries.
Ive always grazed my animals on the common land and Im not going to stop now.
Wikipedia foundation.