What is "ball and socket joint"

Ball \Ball\ (b[add]l), n. [OE. bal, balle; akin to OHG. balla, palla, G. ball, Icel. b["o]llr, ball; cf. F. balle. Cf. 1st Bale, n., Pallmall.]

1. Any round or roundish body or mass; a sphere or globe; as, a ball of twine; a ball of snow.

2. A spherical body of any substance or size used to play with, as by throwing, knocking, kicking, etc.

3. A general name for games in which a ball is thrown, kicked, or knocked. See Baseball, and Football.

4. Any solid spherical, cylindrical, or conical projectile of lead or iron, to be discharged from a firearm; as, a cannon ball; a rifle ball; -- often used collectively; as, powder and ball. Spherical balls for the smaller firearms are commonly called bullets.

5. (Pyrotechnics & Mil.) A flaming, roundish body shot into the air; a case filled with combustibles intended to burst and give light or set fire, or to produce smoke or stench; as, a fire ball; a stink ball.

6. (Print.) A leather-covered cushion, fastened to a handle called a ballstock; -- formerly used by printers for inking the form, but now superseded by the roller.

7. A roundish protuberant portion of some part of the body; as, the ball of the thumb; the ball of the foot.

8. (Far.) A large pill, a form in which medicine is commonly given to horses; a bolus.
   --White.

9. The globe or earth.
   --Pope.

   Move round the dark terrestrial ball.
   --Addison.

10. (Baseball) A pitched ball, not struck at by the batter, which fails to pass over the home plate at a height not greater than the batter's shoulder nor less than his knee (i.e. it is outside the strike zone). If the pitcher pitches four balls before three strikes are called, the batter advances to first base, and the action of pitching four balls is called a walk.

    10. a testicle; usually used in the plural. [vulgar]

11. pl. courage; nerve. [vulgar]

    Ball and socket joint, a joint in which a ball moves within a socket, so as to admit of motion in every direction within certain limits.

    Ball bearings, a mechanical device for lessening the friction of axle bearings by means of small loose metal balls.

    Ball cartridge, a cartridge containing a ball, as distinguished from a blank cartridge, containing only powder.

    Ball cock, a faucet or valve which is opened or closed by the fall or rise of a ball floating in water at the end of a lever.

    Ball gudgeon, a pivot of a spherical form, which permits lateral deflection of the arbor or shaft, while retaining the pivot in its socket.
    --Knight.

    Ball lever, the lever used in a ball cock.

    Ball of the eye, the eye itself, as distinguished from its lids and socket; -- formerly, the pupil of the eye.

    Ball valve (Mach.), a contrivance by which a ball, placed in a circular cup with a hole in its bottom, operates as a valve.

    Ball vein (Mining), a sort of iron ore, found in loose masses of a globular form, containing sparkling particles.

    Three balls, or Three golden balls, a pawnbroker's sign or shop.

    on the ball alert; competent and knowledgeable.

    to carry the ball to carry on the task; to assume the responsibility.

    to drop the ball to fail to perform as expected; to fail to live up to a responsibility.

    Syn: See Globe.

n. (alternative spelling of ball-and-socket joint English)

The ball and socket joint (or spheroidal joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of axes, which have one common center. It enables the bone to move in many places (Nearly all directions).

An enarthrosis is a special kind of spheroidal joint in which the socket covers the sphere beyond its equator.