Here we collect commons definitions and explain the meaning of the word 'commons'.
The commons is not founded on belief. It is based on the ideas and actions of common people doing normal things, which are not common anymore - in capitalism. Here is a collection of commons-definitions.
# Etymology: Commons derives from latin 'cum' + 'munus'. In indoeuropean languages 'munus' is related to a context of gift and mutualization.
On retrouve dans les significatons du terme la double face de la dette et du don. (Dardot/Laval: 22)
Hence, whenever we speak about "commons' a context of 'gifts, contributions and obligations among a certain group of people, or a whole community' is refered to. Sometimes extending to the broader public: the public spectacle of gladiators in Ancient Rome has been called 'gladiatorum munus' in latin. A term that describes a political structure derives from the same idea: 'municipium' -> municipality. Also 'immunitas' (as in parlamentary immumity) points to the obligations a public servant has.
In Dardot and Laval's words: "On comprend surtout, que les termes 'communis', 'commune', 'communia' ou 'communio', tous formés sur la meme articulation de 'cum' et de 'munus', veuillent donc désigner non seulement ce que es 'mis en commun', mais aussi surtout ceux que ont des 'charges en commun'. Le commun, le 'commune' latin, implicque donce toujours une certaine obligation de réciprocité liée à l'excercice de responsabilités publiques' (Émile Benveniste, Vocabulaire des Institutions indo-européennes, vol.1, Minuit, Paris, 1969. p.96-97.) In short, commons always points to the political principle of co-obligation.
The notion of commons helps understanding the double meaning of 'munus': as obligation for and participation in a certain task, activity or endeavour. -> it describes a way of 'acting in common" (agir commun) rather than a function.
The aristotelian term is Κοινόν (koinon) - the Greek "instituting the common" ('mettre en commun') "Ce sont les citoyens qui délibèrent en commun pour déterminer ce qui convient pour la cité et ce qu'il est juste de faire." (Aristotes, Politeia)
"There is no commons without commoning." (attributed to Peter Linebaugh: Zitate)
"commons is the social form of (tangible and/or intangible) matter that is determined by commoning. Hence, a matter only becomes a commons if people predominantly relate to it by commoning and if, therewith, the social form is determined by these very practices." (Euler 2017)
...in response to this question by a member of the CommonsWatch email list: "I desperately need a concise and plain english definition of commons and everything I find is super wordy, complex, academic and not understandable by those not in the field." Julien Lecaille: "A group of people managing something useful for themselves and others, with transparent discussions about this management, without thinking themselves as the only owners and without necessarily turning a profit out of it"
# Stephen Gudeman "The commons is a shared interest or value. It is the patrimony or legacy of a community and refers to anything that contributes to the material and social sustenance of a people with a shared identity: land, buildings, seed stock, knowledge of practices, a transportation network, an educational system, or rituals. As the lasting core, though changeable over time, the base represents temporality and continuity. Without a commons, there is no community; without a community, there is no commons."
# Jon Richter
In general the main argument of the Commons and associated Commoning procedure is the collective control of access and access rights to, but not only, shared resources. For this deliberation we see them in form of: (1) infrastructure, (2) money and (3) information html 
# Silke Helfrich Commons ist ein existentieller Begriff gelingenden menschlichen Daseins.
# Kenneth R. Olwig
"The commons is the material landscape of common lands shared by a community of commonres with customary use rights in the land." (2013:31), pdf