Sirloin steak is a steak cut from the back of the animal.
In U.S. butchery, the steak is cut from the rear back portion of the animal, continuing off the short loin from which T-bone, porterhouse, and club steaks are cut. The sirloin is actually divided into several types of steak. The top sirloin is the most prized of these and is specifically marked for sale under that name. The bottom sirloin, which is less tender and much larger, is typically marked for sale simply as "sirloin steak". The bottom sirloin in turn connects to the sirloin tip roast.
In British and Australian butchery, the word sirloin refers to cuts of meat from the upper middle of the animal, similar to the American short loin.
The word "sirloin" comes from the Middle English surloine, itself derived from the Old French word surlonge, that is, sur la longe 'above the loin'. In Modern French, the cut is called aloyau or faux-filet.
The spelling with "sir" has led to many specious claims that the name comes from the knighting of the cut as Sir Loin, but the "surloine" spelling long predates the "sirloin" spelling, so this is clearly not the origin of the word.