A beef tenderloin steak is a steak cut from any part of the beef tenderloin area except for the small end of it. A tenderloin steak is boneless and about 7 1/2 to 12 cm (3 to 5 inches) in diameter. It can have fat around it, depending on if it is trimmed or not, but otherwise is quite lean.
If the steak is cut from the small end of the tenderloin, then that steak is a “filet mignon“; it should be about 4 cm (1 1/2 inches) in diameter.
Unless otherwise qualified, a “tenderloin steak” means beef tenderloin steak.
A tenderloin steak is very tender, but very lean, so don’t overcook it, as it will dry out quite quickly. And without the fat of other steaks, it doesn’t have as much flavour on its own as do other steaks: restaurants often serve these steaks with sauces to compensate.
Literature & Lore
‘Tenderloin,’ Mrs. [Sarah Tyson] Rorer declares, ‘is absolutely tender, but absolutely tasteless, and therefore undesirable as meat.’ — Jorgenson, Judith. Around The Evening Lamp. Des Moines, Iowa: Des Moines Daily News. 2 June 1896. Page 2.