|leisure = common|
|Using this tag is discouraged, use landuse=* or leisure=*, possibly with designation=common instead.|
|Used on these elements|
|Tools for this tag|
This feature has been labeled as deprecated. The recommended replacement is: landuse=* or leisure=*, possibly with designation=common.|
The reason is documented in Deprecated features. You are still free to continue to use or interpret this tag as you see fit since OpenStreetMap does not have “banned features”. Under no circumstances should you (semi-)automatically change “deprecated” tags to something else in the database on a large scale without conforming to the Automated Edits code of conduct. Any such change will be reverted.
|The verifiability of this tag is disputed. Please see the discussion on the talk page. Please do not remove this message until the dispute is resolved.|
Common land (a common) is land which has an owner, but over which other people have certain rights. Historically these rights typically included the right to graze livestock, collect firewood, or cut turf. Today they include the rights to use the land for certain leisure activities.
In the UK common land is registered. The rights to use it typically include leisure activities such as walking or running, sightseeing, bird watching, picnicking or climbing. Rights would normally not include camping, lighting a fire, or holding an event without the owner's permission. The UK government published a register of common land in 2012, which is archived here - 
- on Wikipedia