Tenderloin is an expensive cut of beef or pork that is taken from along the middle of the back of a cow or pig. The tenderloin is very tender because that particular muscle is not used by the animal, so it never gets toughened up with use. Tenderloins don't have much fat in them, so they aren't as flavourful as many other cuts of meat, but they are perfect for quick cooking in dishes like satay.
Beef tenderloin is an elongated muscle, perhaps two or three pounds, which can be baked whole; in this form it's often called chateaubriand. Because it is mild in flavour, roast tenderloin is often coated with a tasty crust, perhaps made of coarsely ground peppercorns. Beef tenderloin can also be cut crosswise into filet mignon steaks or tournedos.
Tenderloin is also an area in New York City (later also San Francisco) known for vice and corruption. It was named after the choice cut of meat that the police could afford to buy from their immoral earnings from bribes.