(also the name of a book by Benjamin Bratton: The Stack, Software and Sovereignty
"What's a 'stack'? Stacks are lists that are kept of who has asked to speak. Stack managers will call people up in order at the appropriate time. At the Boston GA, for instance, we now use a Group Announcement stack, a Group Proposal stack and an Individual Stack. There are mini-stacks kept when someone is speaking. Each person is allowed to speak without interruption. If someone has a clarifying question (very important qualifier, there) or a directly relevant point of information, they make a gesture. A Floor Time Manager will put them on a list to speak when the current speaker is finished. If the Floor Time Manager determines that the question is for purposes of clarification or the point of information is not directly relevant, the person can opt to be put on the individual stack. No one is denied the opportunity to speak.
It should be noted that in NY and in Boston, we use a tool called 'progressive stack'. The stack manager watches to see that a plurality of voices are being heard. If one demographic is being heard too often, the stack manager has the discretion to move someone up the stack who might represent a different demographic. We've most typically seen this be based on gender. More men put themselves on stack to speak than women. We might hear from 5 men in a row and the stack manager would then bump a woman up the stack. As we get to know each other better the progressive stack management will likely get more refined so that more marginalized voices are bumped up the stack more often." (http://www.dailykos.com/story/2011/10/08/1022710/--occupywallstreet:-a-primer)