Battery Grip
An accessory that bolts to the base of a camera to hold additional batteries and duplicate the controls for vertical shooting.
In depth
The obvious benefit is runtime, but the ergonomic one usually matters more: a duplicated shutter release and command dials mean the wrist is not twisted upward for every portrait-orientation frame. The extra mass also steadies long lenses, at the cost of bulk that makes the camera less discreet.
Examples in practice
Shooting a full-day event in portrait orientation, a grip removes the shoulder strain of a raised elbow on every frame.