Red meat is any meat that is dark-colored before cooking, such as beef, lamb, venison and mutton. The meat of most adult mammals falls in the category of red meat. Another, more unusual, example of red meat is from ostriches.
The most important factor that makes a red meat, as opposed to a white meat, is whether the animal's muscles are mostly fast-twitch or mostly slow-twitch. Slow-twitch muscles are used often and for extended activities, such as constant walking, standing or flying. Slow-twitch muscles store more of the protein myogoblin, which is reddish in color, than fast-twitch muscles. Red meat is made out of slow-twitch muscles.