Common sense is defined by Merriam-Webster as, "sound and prudent judgment based on a simple perception of the situation or facts." Thus, "common sense" (in this view) equates to the knowledge and experience which most people already have, or which the person using the term believes that they do or should have.
The Cambridge Dictionary defines it as, "the basic level of practical knowledge and judgment that we all need to help us live in a reasonable and safe way".
