In the culinary arts, the term filet is used to indicate a boneless cut of meat, usually a tender, high-quality one. So in that sense, the word filet is a generic term, not specific to any particular cut of meat.
Nevertheless, the word filet is sometimes used as a shorthand for filet mignon, which are small boneless steaks cut from the front end of a beef tenderloin.
The word filet can also be used as a verb, in which case it refers to the act of slicing a larger primal cut into individual boneless steaks.
The word "filet" is pronounced "fill-AY" or "fee-LAY." And yet, in some countries, the word for a boneless cut of meat is spelled "fillet," with two Ls, and pronounced just like it is written: "FILL-et."
This can lead to some confusion sometimes because the word fillet is often used to specify a boneless cut of fish, or as a verb, to refer to the act of slicing the boneless side off of a fish. In the U.S., however, it's filet when speaking of meat, and fillet when speaking of fish.
So What is Filet Mignon, Anyway?
This might be a more important question anyway, although as with so much culinary terminology, there are no hard and fast answers here either.
But to begin with, it helps to picture a beef tenderloin. It's a long, pencil-shaped muscle situated literally within the loin of the beef carcass. And because it extends across two common subprimals, the short loin and the sirloin, butchers have to choose whether to leave it in, in which case it is included in the T-bone and Porterhouse steaks cut from that loin; or removed and fabricated into individual tenderloin steaks and roasts.
When it's removed, it can be sold whole, sliced into individual steaks, or a combination of steaks and larger roasts.
Now, steaks that are cut from the pointy end of the tenderloin are called filet mignon. In French, the word mignon refers to something that is small, dainty or cute. And the tiny little steaks that are cut from the pointy end of the tenderloin are certainly dainty.
They're also exceedingly tender. Note that the actual tip of the tenderloin, the point itself, as it were, can't be made into a steak, so in practice it usually ends up as either ground beef or stew meat. But if you ever have a whole tenderloin of your own, you could grill it up and serve it on a skewer as an hors d'oeuvre.
But back to the filet. The first few steaks from the tip of the tenderloin are properly called filet mignon. The middle section is where we get chateaubriand, and if sliced into steaks you could call them tenderloin steaks, but not technically filet mignon.
Finally we get to the eraser end of the pencil, known as the butt end of the tenderloin, and from it we also get tenderloin steaks, but because the butt end consists of a few separate muscles, you'll sometimes see these steaks tied with butcher twine to hold them together. When you see that, it's a sign that the steak in question is not a true filet mignon. That doesn't mean it won't be good, just that it's from the opposite end of the tenderloin.