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. a False Controversy can happen when one mistakes something to not be in consensus when in fact it is. Claiming false controversies is a common way of creating uncertainty and doubt.
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, Bayesianism over frequentist probability (and more to be added)