A "meat bird" is a breed developed especially for meat production, and has the following characteristics: they grow and feather rapidly; they are broad-breasted; they are deep-bodied; their heart girth (i.e. their circumference just behind the wings) is large; and their backs are wide and flat (the same width front to back, not tapered). Breeds originally developed for meat production include the Brahma, Cochin and Cornish.
There are some "dual purpose" breeds (good for eggs and meat though not as efficient at either as a specialized breed) that have some potential as efficient meat producers. These include the Delawares, New Hampshires, and Plymouth Rocks.
Finally, there have been a few hybrids developed for meat production that are even more efficient than the specific meat breeds. Most notable is the Cornish X Rock hybrid, a cross between a White Cornish male and a White Plymouth Rock female. Broad-breasted, deep-bodied, and fast-growing, they have the potential to dress out at five to six pounds at eight weeks of age. An added plus is that they are white feathered to facilitate clean picking.
There are also a few hybrids developed for pasture production. They grow a little slower than the Cornish X Rock, but because of this the meat is more flavorful. (FYI - with chickens age is directly proportional to flavor. Unfortunately age is also directly proportional to toughness!)
Hopefully these added "specifics" help clarify "meat matters" as applied to chickens!