The force experienced by a pilot or astronaut when the craft in which he or she is travelling accelerates or decelerates rapidly. The unit g denotes the acceleration due to gravity, where 1 g is the ordinary pull of gravity.

Early astronauts were subjected to launch and re-entry forces of up to 6 g or more; in the space shuttle, more than 3 g is experienced on lift-off. Pilots and astronauts wear g-suits that prevent their blood pooling too much under severe g-forces, which can lead to unconsciousness.