Consensus

Consensus process is a way of running a business meeting. Consensus requires that every member of the business meeting consent to an item of business to pass it. If one person blocks consensus, that item is not passed.
History
Consensus was widely practiced in the Americas before Columbus’s arrival.
John Woolman, a Friend, lived with eastern Pennsylvania Indian tribes for a period. Around that time, Friends started to use consensus process for their business. Friends formalized their type of consensus process and added a number of innovations to the process.
The American Friends Service Committee taught consensus process to numerous antiwar and antinuclear protestors. From these AFSC training sessions, consensus process has spread to cooperatives, to environmental organizations and beyond.
Consensus Process
In Quaker consensus, a clerk runs the meeting. The clerk decides who shall speak next. 
Typically, a business meeting runs from an agenda that has been agreed to early in the meeting. Each item of business is brought forth in turn and discussed. Any resolutions, called minutes, are formulated and passed. If the clerk sees that no resolution is possible due to one or more members of the business meeting disagreeing, the clerk passes on to the next item in the order of business.
All minutes that are passed, and other significant sentiments within the meeting, are written down. Typically, a second clerk, named the recording clerk, records the minutes and other information.
In formal Quaker business practice, the minutes are read one more time after a few minutes have been recorded, and the meeting must approve that this was what transpired. 
One or more members may block a relatively popular proposal. This is sometimes referred to as standing in the way. Alternatively, one or more members may have quibbles with a proposal but allows the proposal to pass. This is sometimes referred to as standing aside. A problem with a proposal is called a concern.
Large business meetings such as yearly meeting sessions have, in addition to a presiding clerk and a recording clerk, one or two reading clerks and an epistles clerk. The reading clerks read texts to the assembled meeting. The epistles clerk sends letters. Often yearly meetings send a fairly lengthy epistle to other yearly meetings at their concluding session.
Large business meetings benefit from microphones, as some members are hard of hearing. Wireless microphones can be easily passed down a row of seats to a speaker, or speakers can walk to a fixed microphone.
In practice, clerks almost always let visitors of good will have their say in a business meeting. However, requiring membership in Quaker meetings is a reasonable safeguard against lone disruptors causing havoc with the meeting’s consensus process.
Variants to consensus process:
In large meetings especially, the clerk often will leave 15 seconds or more of silence between speakers. If nothing else, this allows people to properly digest what the previous speaker said. It also fosters a sense of worship within the business meeting. At other times a clerk may ask the Quaker business meeting to drop into worship for five or ten minutes. These techniques are especially useful when debates are heated.
One way to help solve an intractable question is to move the proponents and opponents on a proposal off to another, smaller meeting, called a threshing session.
Non-Quaker variants to consensus
I have seen a consensus process meeting with 120 participants, the 1979 Clamshell Congress in Providence, RI, which used the following elements:
-a presiding clerk
-a recognizing clerk, in charge of saying who would speak next, second, and third
-a person at a blackboard, in charge of keeping lists of names of who wanted to speak next
-two runners, who took names from the audience and ran them down to the blackboard
–a timekeeper, charged with keeping everyone’s message to 1 minute or less.
–a vibes watcher, charged with monitoring the energy of the meeting’s members. 
–two process people, charged with discussing process questions off of the floor. If a meeting member had a process question unrelated to the issue at hand, for example, “can we break for lunch now”, that member would have to convince one of the two process people that the process question was legitimate. If one or both process people were so convinced, the idea would be passed to the presiding clerk for discussion.
Consensus, like any form of government, is a tool. Poor tools can often be improved.
Strengths of consensus.
When done properly, consensus process produces uniformly wiser decisions than, say, Robert’s Rules of Order. Consensus process also fosters unity.
Problems with consensus
At times strong advocates of a certain position will be medically unable to attend a meeting, or late, or tied up. The other members should take care to properly represent missing people as if they had attended. In general, consensus process is about seeking unity.
Consensus can be a long process. It uses a great deal of energy. Particularly in change-a-lightbulb questions, consensus process is unnecessary. Consensus also flounders in highly technical areas where the membership isn’t skilled in that subject. 
Consensus process gets a bit more confusing when more than one question is on the floor. A good clerk needs the discipline to move all but one question off the floor temporarily, and focus on that one question for a time.
Consensus process has historically bogged down over issues of faith, over issues of money and over personality disputes. Faith is something that doesn’t change much with talking, and people come in with hardened opinions about money. Friends are cautioned to stay out of long heats and arguments.