Common knowledge: information that is so generally accepted and commonly known that it can’t be claimed to be the idea or property of one person or source.
These statements are common knowledge:
The chemical formula for water is H2O.
The sky is blue.
These statements are not common knowledge and must therefore be referenced:
The University of Bristol spends £2,300 a day on water. (University of Bristol, 2008)
The sky is blue due to a process called Rayleigh scattering. Short wavelength light, which is blue in colour, is absorbed by gas molecules in the atmosphere and then scattered across the sky, so the sky looks blue. (Science Made Simple, 1997)
Beware. The internet can confuse understanding of what is common knowledge because so many sites borrow from each other and do not reference their sources properly. Remember, information on web pages cannot automatically be treated as common knowledge, even if there is no citation. If you’re in any doubt, always provide a reference!