Frances Frei and Anne Morriss at HBR.org offer the best definition of organizational culture:
culture guides discretionary behavior and it picks up where the employee handbook leaves off. Culture tells us how to respond to an unprecedented service request. It tells us whether to risk telling our bosses about our new ideas, and whether to surface or hide problems. Employees make hundreds of decisions on their own every day, and culture is our guide. Culture tells us what to do when the CEO isn’t in the room, which is of course most of the time.
How do people act when you (or your boss) isn’t in the room? How do you act?