The word spatchcocking refers to the process of removing the backbone and breaking the breastbone of a bird. The partially boned fowl can then be spread open and flattened for cooking. While similar to butterflying, the process of spatchcocking implies the use of whole poultry rather than a single cut of meat. It is believed that the word is derived from the term "dispatch the cock."
Spatchcocking a turkey, chicken, or any other poultry results in a bird that takes up less oven space, cooks more evenly and quickly, and is easier to carve.
Source: Flickr User thebittenword.com