What a group holds in common without having to say. Culture is the part of a system that is never written down.
Latin cultura (cultivation, tending), from colere (to till, tend, cultivate); what a group tends and holds in common without needing to say it aloud.
Latin colere is to till, and culture was first agricultural before it was social, something tended steadily rather than declared once. What a group holds this way was grown over time, not authored, which is exactly why no one wrote it down.