Common knowledge refers to widely believed statements of fact; that is, it is knowledge shared by most members of a community.
For example, throughout the 20th century, most Canadians were likely aware that the penny depicted a maple branch, since they each used the penny coin on a near daily basis (Royal Canadian Mint, n.d.-a). Since Canadians no longer use the penny coin as part of their currency, the belief that the coin depicts a maple branch is no longer as widely shared as it once was (Royal Canadian Mint, n.d.-b).
In this way, what counts as common knowledge for a community in the 21st century is different than what counted as common knowledge for a community in the 20th century, even within the same geographical region. Since common knowledge is determined in reference to a specific community, what counts as a case of common knowledge for one community may need be explained to members of other communities.
To help your audience understand your writing, please cite sources for facts that may not be known by the audience of your work. For guidance on correctly integrating professional experience that is common knowledge within your field, please see Citing Common Knowledge and Lived Experience (University of Manitoba) and Using Evidence: Common Knowledge and Personal Experience (Walden University).
References
Royal Canadian Mint. (n.d.-a). A national symbol--the 1-cent coin. https://www.mint.ca/store/mint/about-the-mint/1-cent-5300004
Royal Canadian Mint. (n.d.-b). Phasing out the penny. https://www.mint.ca/store/mint/about-the-mint/phasing-out-the-penny-6900002
