common knowledge

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun What "everybody knows", often with reference to a specific community; something which cannot reasonably be contested.
• noun logic, game theory A special kind of knowledge for a group of agents, such that when all the agents in a group G know p, they all know that they know p, they all know that they all know that they know p, and so on ad infinitum.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun anything generally known to everyone