The Collaborative International Dictionary of English v.0.48:

taxicab \tax"i*cab\, n.
   an automobile with a professional driver which can be hired
   to carry passengers; -- also called a taxi, and informally
   called a cab or a hack. The driver of a taxicab is
   referred to as a cab driver or cabbie, and sometimes as a
   chauffeur or hackie.

   Note: Taxicabs may be engaged by a prior appointment made,
         e.g. by telephone, or they may cruise for passengers,
         i.e. they may drive in city streets and stop to pick up
         pasengers when they are signalled by a prospective
         passenger. The act of signalling a taxicab (usually by
         a wave of the arm) is often called

   to hail a cab or

   to flag down a cab.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Cab \Cab\ (k[a^]b), n. [Abbrev. fr. cabriolet.]
   1. A kind of close carriage with two or four wheels, usually
      a public vehicle. "A cab came clattering up." --Thackeray.
      [1913 Webster]

   Note: A cab may have two seats at right angles to the
         driver's seat, and a door behind; or one seat parallel
         to the driver's, with the entrance from the side or
         front.
         [1913 Webster]

   Hansom cab. See Hansom.
      [1913 Webster]

   2. The covered part of a locomotive, in which the engineer
      has his station. --Knight.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cab \Cab\ (k[a^]b), n. [Heb. qab, fr. q[=a]bab to hollow.]
   A Hebrew dry measure, containing a little over two (2.37)
   pints. --W. H. Ward. --2 Kings vi. 25.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cab
    n 1: a compartment at the front of a motor vehicle or locomotive
         where driver sits
    2: small two-wheeled horse-drawn carriage; with two seats and a
       folding hood [syn: cab, cabriolet]
    3: a car driven by a person whose job is to take passengers
       where they want to go in exchange for money [syn: cab,
       hack, taxi, taxicab]
    v 1: ride in a taxicab [syn: taxi, cab]

Easton's 1897 Bible Dictionary:

Cab
   hollow (R.V., "kab"), occurs only in 2 Kings 6:25; a dry
   measure, the sixth part of a seah, and the eighteenth part of an
   ephah, equal to about two English quarts.