A Consensus is a general or full agreement between members of a group. A consensus can be useful in deciding what's true (e.g, a scientific consensus), or as a criteria in decision making. A False Consensus can happen when someone thinks a position is in consensus when it isn't. one can also claim a consensus falsely to advance their position and make it difficult for others to oppose it.
There are many things that are considered a consensus on LessWrong, even though they're are not considered a consensus in the scientific community, such as: One-Boxing, cooperating on the Prisoner's Dilemma (and more to be added)
Related Pages: Disagreement, Modesty, Modesty argument, Aumann agreement