The power to have a statement accepted into the Shared Imaginary Space (or diegesis). In other words, whether what you say is "real" within the game-world. For example, a player may say "I chop his head off" -- but the statement doesn't have credibility until the GM confirms by saying "Indeed, you chop his head off." Coined by Vincent Baker (aka Lumpley) on the Forge as part of The Lumpley Principle.

References:

• Vincent's standard rant: power, credibility, and assent
• Player power abuse