The Flip Meter is a meter in Super Paper Mario. When Mario uses his Flip move, it will start decreasing. When the meter is empty, Mario will loose 1 HP. If Mario flips to 2D, then the meter will slowly start getting the points it lost back. The Flip Meter only appears in 3D or when it's getting its points back.
When he defeats the boss at the bottom of the Pit of 100 trials, the meter will never decrease.