Battery grip
A battery grip is an accessory for an SLR/DSLR (and occasionally other cameras), which allows the the camera to hold multiple batteries to extend the battery life of the camera, and adds a vertical grip with an extra shutter release (and other controls), facilitating the shooting of portrait photography.[1]
It usually attaches to the camera body through the cameras own battery compartment and provides a cassette to hold additional batteries to increase the battery life for the camera. Most battery grips also come with a second cassette allowing the photographer to shoot using multiple AA batteries.
Battery grips are usually designed to fit only one or a few specific camera models, since they must match the body's shape, connectors, and power requirements.