The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A spherical body of any substance or size used to play
      with, as by throwing, knocking, kicking, etc.
      [1913 Webster]

   3. A general name for games in which a ball is thrown,
      kicked, or knocked. See Baseball, and Football.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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      inking the form, but now superseded by the roller.
      [1913 Webster]

   7. A roundish protuberant portion of some part of the body;
      as, the ball of the thumb; the ball of the foot.
      [1913 Webster]

   8. (Far.) A large pill, a form in which medicine is commonly
      given to horses; a bolus. --White.
      [1913 Webster]

   9. The globe or earth. --Pope.
      [1913 Webster]

            Move round the dark terrestrial ball. --Addison.
      [1913 Webster]

   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.
       [Webster 1913 Suppl. +PJC]

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

   11. pl. courage; nerve. [vulgar]
       [PJC]

   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.
      [1913 Webster]

   Syn: See Globe.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "on the ball":
   advertent, agile, agog, alert, alive, all ears, all eyes,
   assiduous, attentive, awake, aware, bright, careful, clear-sighted,
   clear-witted, clearheaded, concentrated, conscious, diligent,
   earnest, finical, finicking, finicky, heedful, intense, intent,
   intentive, keen, meticulous, mindful, nice, niggling, nimble,
   observant, observing, on the, on the alert, on the job, open-eared,
   open-eyed, openmouthed, prompt, qui vive, quick, ready, regardful,
   sharp, sleepless, smart, unblinking, unnodding, unsleeping,
   unwinking, wakeful, watchful, wide-awake