Information considered to be "common knowledge" does not need to be cited (unless you are directly quoting an author). If you are unsure, err on the side of caution and cite the source. "Common knowledge" refers to anything that you expect the readers to already know. An example of general common knowledge is that George Washington was the first U.S. president.
Common knowledge can also be field-specific. For example, a nursing student would not have to cite a basic definition of systolic and diastolic blood pressure, since students and instructors in that field are familiar with the concept already. Here are a few more examples:
Common Knowledge:
- The sky is blue.
Not Common Knowledge:
- The physics of light refraction explains why the sky appears blue.