Bottom sirloin is a cut of beef — part of the sirloin primal cut. Primal cuts are large divisions of the cow, and the sirloin primal cut is a section that runs from the spine down to about the middle of the body above the flank just in front of the rear leg. Bottom sirloin is the source of several sub-primal cuts including ball tip sirloin, tri-tip, and flap cuts. This type sirloin is known by several other names including sirloin butt and thick flank. The name is primarily used in the United States. In Britain, beef is traditionally divided into slightly different primal cuts, and the British sirloin is more equivalent to another American primal cut called the short loin.
The primal cut of sirloin is divided into three main sections, the top sirloin, the tenderloin, and the bottom sirloin. These cuts are all popular and contain some of the most desirable cuts used for steak, such as chateaubriand, as well as some less desirable, but still high quality, steaks that are very popular. Sirloin is found at almost every steakhouse and American restaurant, and many establishments that serve only one or two steaks are likely to have a sirloin on their menu. In most restaurants, sirloin is sold as plain sirloin or top sirloin. If a steak is not specifically designated as top sirloin, it is likely that it comes from the bottom sirloin.
As a subprimal cut, this sirloin is traditionally further divided into three smaller cuts. The ball tip is popular for steaks, and is the source of many such offerings at restaurants. If a menu advertises sirloin, it is most likely from this cut. Tri-tip, often called sirloin tip, is many times used as a roast or for barbecue but may be grilled like other steaks as well. This cut of meat is often cooked slowly for long periods in order to achieve tenderness.
The flap portion of the bottom sirloin is an extension of the flank and is rather tough, so it is rarely used for steak. It is usually ground up for hamburger or sold as stew meat. Sometimes whole flap steaks are used in the same way as flank or skirt steaks and are slow cooked or braised. These cuts, cooked slowly and thinly sliced against the grain, are suitable for dishes like fajitas and barbecue.