Cambridge - basic level practical knowledge and judgement.
Wikipedia - basic ability to perceive, understand, and judge things, shared by and reasonably expected of nearly all people without need for debate.
MeWebster - the ability to think and behave in a reasonable way and to make good decisions; sound and prudent judgment based on a simple perception of the situation or facts.
sample useage: You really should go to see a doctor if your leg hurts that much. It's just common sense!