The cut of beef, pork, veal or lamb that comes from the underside of the loin (the tenderloin). Filet Mignon is traditionally cut from the center of the tenderloin and grilled or broiled and is sometimes pan grilled or pan fried. If the whole center cut is roasted, it is called Chateaubriand. This was probably a dedication to the Vicomte de Chateaubriand by his chef Montmireil.
The tapered end of the tenderloin is cut into smaller steaks and these are known as tournedos.
Filets of fish are cuts made but slicing the flesh away from the bone to produce a flat thin slice of fish. There are knives (fish knife, filet knife, boning knife) designed for just this purpose. They have long, thin, flexible blades that help in slicing clean filets.