Butt plug, also known as anal plug. This is a device that is pushed through the anus into the rectum. Most have a diameter sufficient to feel interesting when inserted, then a reduced diameter, and finally a large diameter at the distal end. This last section is to keep the plug from going all the way into the rectum where it could be lost.
Both men and women enjoy wearing butt plugs while masturbating, sometimes for hours on end. The feeling of sitting down once an anal plug is inserted all the way, is erotic.
Butt plugs are usually made of silicone or similar soft, rubbery material.
Some anal plugs have a big enough base that they can stand upright when placed on a floor or table. A few have suction cup bases that can be attached to a wall. The purpose of this configuration is to slowly sit down, thereby inserting the plug.
The size of an anal plug can be quite astounding. Experienced users can sometimes take a plug several inches in diameter.
Length too, can be interesting. While most plugs are under 10 inches (25 cm), they can be as long as a yard (meter) or more. These are smaller in diameter, and very flexible. They are no longer just plugs, but a device that can be insert well into the large intestine for a very deep stimulation.