Tenderloin applies to the entire tenderloin strip, while a filet mignon is a slice from short loin. The short loin is the small end of the tenderloin.
Filet mignon is considered the king of steaks, and depending on the grade, can be quite expensive when dining out. Filet mignon slices are generally small, thick cuts of the beef tenderloin. The short loin is located on the back rib cage of the cow. Because this area is not weight-bearing, it keeps the connective tissues from being toughened by the regular physical activity of the animal. This results in the extreme tenderness of this cut of meat.