• The P2P Foundation offers an extensive article on the definition, meaning and etymology of the term “commons”.
• Wikipedia on commons has a more restricted view.
• Commoners Co-op offers a short helpful view: A Definition of the Commons

My own synthesis, mostly in line with what has been published by David Bollier and Silke Helfrich is that a commons implies

• a shared, valued or valuable resource (of any kind, material or immaterial)
• a community of people (commoners) who collectively use or work with that resource, and who also either own, or govern, or steward that resource
• structures, conventions, rules etc. agreed within the community by which the members of the community govern or steward the resource

Elinor Ostrom set out the principles that appear to be key to effective material commons.

• here they are set out concisely on the “Patterns of Commoning” site.

• narrower→ a knowledge commons is a particular variety of commons with particular characteristics that distinguish it from a physical commons.
• grammar-related→ commoning is the process and/or practice of setting up and curating, governing or stewarding that resource.