Silverside is a cut of beef from the hindquarter of cattle, just above the leg cut. It gets its name because of the "silverwall" on the side of the cut; this is a long fibrous "skin" of connective tissue, which has to be removed as it is too tough to eat. The primary muscle is the biceps femoris.
Silverside is boned out from the top along with the topside and thick flank. It is a 2nd class roasting joint, or may be sliced for minute steak or beef olives, or split in two to produce a salmon-cut. In most parts of the U.S., this cut is known as outside or bottom round; it is also known as a rump roast; traditionally, a hindquarter is laid on the cutting table with the outside down or to the bottom, as opposed to the inside being on top.
In Australia, Ireland and New Zealand, silverside is the cut of choice for making corned beef, so much so that the name "silverside" is often used to refer to corned beef rather than any other form of the cut.