Alt-pause is a key combination for FS2_Open that allows a player to pause the game, move the camera around and take screenshots.
Note that while you are in alt-pause mode, some of the keys do different things
Enter - External view, same as Numpad . in normal gameplay
Tab - Disable HUD, same as Shift-O in normal gameplay
Numpad * - Select next target, same as T in normal gameplay
Numpad / - View from target