The definition of Common Good:
noun
the advantage or benefit of all people in society or in a group:
In spite of our differences, we shall work for the common good.
source: http://www.dictionary.com/browse/common-good
Another definition that describes this term that is used very often:
"In philosophy, economics, and political science, the common good (also commonwealth, common weal or general welfare) refers to either what is shared and beneficial for all or most members of a given community, or alternatively, what is achieved by citizenship, collective action, and active participation in the realm of politics and public service."
Source: https://en.wikipedia.org/wiki/Common_good
Basically the defintion could be summed up in this one phrase/saying:
"United we stand, divided we fall!"