A practice where an entire agile team works on the same story for a period of time.

http://agiletools.wordpress.com/2007/12/03/what-is-swarming/

http://www.mountaingoatsoftware.com/blog/should-a-team-swarm-on-to-one-backlog-item-at-a-time

http://www.infoq.com/news/2013/02/swarming-agile-teams-deliver