Boss

An extension or strengthened section or projection (usually cylindrical) that holds the end of a pin or shaft. For example, the holes in the piston through which the piston pin is placed would easily break the thin walls of the piston when under pressure. The area around the hole (on the inner side of the piston) are strengthened to prevent breakage. This area is the piston boss.
See also brake boss, gudgeon pin boss, horn boss, piston pin boss.