The Tenderloin comes from an elongated psoas major muscle, which is lean and tender.
Beef tenderloin is widely regarded as the most tender cut of beef. It’s a portion of the ever-popular T-bone or porterhouse steak, and the upscale filet mignon comes from it as well. These tender steaks do well with a quick stove-top sear before getting finished in the oven.
Beef tenderloin is a larger cut of beef that includes the filet mignon. If you want to cook a large, shareable beef dish like beef Wellington, purchase the entire tenderloin (or a large piece of it).