Culture is best defined as "values plus behavior" and is often described as "how employees act when no one is looking."
Culture is such an important part of your business. It's really the foundation of the organization.
I've previously defined culture as the set of values and norms that guides how the business operates; culture happens when we operationalize the values.
Herb Kelleher's definition of culture is still my favorite: Culture is what people do when no one is looking.