"Flexible Raid is a new difficulty, added in patch 5.4.0 for World of Warcraft, that sits between Raid Finder and Normal difficulty that will be available for pre-made groups of 10–25 players, including any number in between. That means if a player have 11, 14, or 23 other people available for a Raid, they'll all be able to participate, and the enemies will scale dynamically."

That is the definition of Flex Raid for those of you who don't know.