A hanger steak is a cut of beef steak which is said to "hang" from the diaphragm of the steer. (Anatomically the diaphragm is one muscle, but it is commonly cut into two separate cuts of meat: the "hanger steak" traditionally considered more flavorful due to its proximity to the kidneys, and the outer skirt steak which is composed of tougher muscle within the diaphragm.) The hanger is attached to the last rib and the spine near the kidneys. It resembles flank steak, and is a vaguely V-shaped pair of muscles with a long, inedible membrane down the middle. The hanger steak is not really tender, but has a lot of flavor, and is best marinated and cooked quickly over high heat (grilled or broiled) and served rare or medium-rare, to avoid toughness. Chefs with experience preparing beef kidneys report that the hanger steak's aroma preserves a trace of kidney.
There is only one hanger steak per animal, and the entire cut typically weighs about 1 to 1.5 lbs (450 to 675g). It is prized for its flavor, and was sometimes known as "butcher's steak" because butchers would often keep it for themselves rather than offer it for sale. That may just be because there was never demand for it due to the finicky cut (which may take experience to cook). If no one else will purchase it, the business-minded butcher would take the cut home himself.
The hanger steak has traditionally been more popular in Europe. In French, it is known as the onglet, in Italian the lombatello, and in Spanish the solomillo de pulmon. In the United States, it is slowly starting to become popular; formerly, it was not separated as an individual cut. Even today it is usually ground into hamburger in the US.
It is also known as the "hanging tender", and occasionally is seen on menus as a "bistro steak".