In engineering, a boss is a protruding feature on a workpiece.

A common use for a boss is to locate one object within a pocket or hole of another object. For instance, some motors

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

use a precisely machined boss on the front face to locate it on the mating part.

The term 'boss' when used in engineering can also relate to a finishing edge around (usually) a circular opening that allows the opening to locate onto, or within another opening thus locating or joining two items together with a view to the location or joining being temporary or semi-permanent.
A common everyday example of a boss is the housing of the rotation spindle in a washing machine drum, or on a cylinder lawn mower at the end of the cutting blade cylinder which may house a bearing set to allow the cylinder to rotate through one plane, but held firm in another plane.

A boss can also be a brass eyelt on a sail, it is a generic term to describe an item designed to facilitate the use with, within, on or around another item whereby one cannot operate properly without the other.

Boss
A projection or an enlarged section of a casting through which a hole may be machine.