The term "common"
Strictly speaking, the term 'common' as used here is inaccurate.
The coiner of the expression Tragedy of the Commons, ecologist Garrett Hardin, came to realise this, and later said '...the weightiest mistake in my synthesizing paper was the omission of the modifying adjective "unmanaged"'. However, 'unmanaged commons' is probably an oxymoron.
Correct terms for the concept of 'common' in this context include 'open access resource' and 'un-owned resource'. However, in informal use the term 'common' is taken to have this meaning, and this is what it means in the expression Tragedy of the Commons.