Steak
A steak (from Old Norse steik, "roast") is generally a cut of meat or fish cut perpendicular to the muscle fibers, or of fish cut perpendicular to the spine. Meat steaks are usually grilled, pan-fried, or broiled, while fish steaks may also be baked.
Steak can also be meat cooked in sauce (cf. steak and kidney pie) or minced meat formed into a steak shape (cf. Salisbury steak, hamburger steak).
Without qualification, the word "steak" generally refers to beefsteak. Steaks from other animals are usually qualified as, e.g., 'swordfish steak' or 'venison steak'.
Beefsteak
The more tender cuts of beef, from the loin and rib, are cooked quickly, using dry heat, and served whole. Less tender cuts from the chuck or round are cooked with moist heat or are mechanically tenderized (cf. cube steak). Steak can be cooked to a level of very rare (bleu, a cold raw center), rare, medium rare, medium, medium well done, or well done. Beef, unlike certain other meats, does not need to be cooked through. Food-borne human illnesses are not normally found within a beef steak, though surfaces can potentially be contaminated from handling, and thus, very rare steak (seared on the outside and raw within) is generally accepted safe.
Fish steaks
For fish, steaks are cut perpendicular to the spine and include bones. In North America, fish usually cooked as steaks include swordfish, halibut, and tuna. Other fish often cooked as steaks include salmon and mahi-mahi, though they are also frequently cooked as fillets or whole.
Unlike beefsteaks, fish steaks are often baked in sauce.