A way to order your meat. Typically applied to beef, especially steak. I suppose it might be possible to order other meats in this manner, but I wouldn't recommend it. Essentially this is a step below rare and more or less bordering on raw. For all of you who've had to resort to asking for your meat to be cooked as little as they possibly can, or "so rare that it moos" this is likely what you want. The outside will be slightly brown, but with an inside ranging from light pink to deep red with a cold center.
I'm told that a blue steak tastes more or less like DIY sashimi arriving in one big lump.