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A battery grip is an accessory for an SLR/DSLR (and occasionally other cameras), which allows 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.
It usually attaches to the camera body through the camera's 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.