The Collaborative International Dictionary of English v.0.48:

Tank \Tank\ (t[a^][ng]k), n.
   A small Indian dry measure, averaging 240 grains in weight;
   also, a Bombay weight of 72 grains, for pearls. --Simmonds.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tank \Tank\ (t[a^][ng]k), n. [Pg. tanque, L. stangum a pool; or
   perhaps of East Indian origin. Cf. Stank, n.]
   1. A large basin or cistern; an artificial receptacle for
      liquids.
      [1913 Webster]

   2. A pond, pool, or small lake, natural or artificial.
      [Webster 1913 Suppl.]

            We stood in the afterglow on the bank of the tank
            and saw the ducks come home.          --F.
                                                  Remington.

            The tanks are full and the grass is high. --Lawson.
      [Webster 1913 Suppl.]

   3. (Mil.) a heavily armored combat vehicle which moves on
      caterpillar treads, rather than wheels. It typically
      carries a cannon and a heavy machine, and sometimes other
      weapons. It is the main distinguishing weapon of an
      armored division.
      [PJC]

   4. a jail cell for temporarily holding prisoners, as in a
      police station.
      [PJC]

   Tank engine, a locomotive which carries the water and fuel
      it requires, thus dispensing with a tender.

   Tank iron, plate iron thinner than boiler plate, and
      thicker than sheet iron or stovepipe iron.

   Tank worm (Zool.), a small nematoid worm found in the water
      tanks of India, supposed by some to be the young of the
      Guinea worm.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tank
    n 1: an enclosed armored military vehicle; has a cannon and
         moves on caterpillar treads [syn: tank, army tank,
         armored combat vehicle, armoured combat vehicle]
    2: a large (usually metallic) vessel for holding gases or
       liquids [syn: tank, storage tank]
    3: as much as a tank will hold [syn: tank, tankful]
    4: a freight car that transports liquids or gases in bulk [syn:
       tank car, tank]
    5: a cell for violent prisoners [syn: cooler, tank]
    v 1: store in a tank by causing (something) to flow into it
    2: consume excessive amounts of alcohol
    3: treat in a tank; "tank animal refuse"