Flip is a cardinal action in VVVVVV.

It is performed by pressing a simple action button, and will make Viridian flip his gravity to be in the opposite direction to where it was last, with two types of gravity being "up" and "down".
Some other factors may change gravity forcefully; Teleporting will set gravity to "down", Dying and respawning will set the gravity to whichever direction the spawn point is at, and Gravity Lines will force a flip regardless of current orientation.
In Flip Mode, the entire game will be flipped upside down, meaning that all "down" directions become "up" and vice-versa.