The Collaborative International Dictionary of English v.0.48:

pit stop \pit" stop`\ (p[i^]t" st[o^]p`) n.
   1. (Auto Racing) A stop by one of the competing cars in a pit
      by the side of the racetrack, to take on gasoline, change
      tires, or perform other maintenance.
      [PJC]

   2. Hence: Any pause in a journey, to eat or drink, or to use
      a rest room.
      [PJC]

   3. A place where one may make a pit stop[1 or 2].
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Pit \Pit\, n. [OE. pit, put, AS. pytt a pit, hole, L. puteus a
   well, pit.]
   1. A large cavity or hole in the ground, either natural or
      artificial; a cavity in the surface of a body; an
      indentation; specifically:
      (a) The shaft of a coal mine; a coal pit.
      (b) A large hole in the ground from which material is dug
          or quarried; as, a stone pit; a gravel pit; or in
          which material is made by burning; as, a lime pit; a
          charcoal pit.
      (c) A vat sunk in the ground; as, a tan pit.
          [1913 Webster]

                Tumble me into some loathsome pit. --Shak.
          [1913 Webster]

   2. Any abyss; especially, the grave, or hades.
      [1913 Webster]

            Back to the infernal pit I drag thee chained.
                                                  --Milton.
      [1913 Webster]

            He keepth back his soul from the pit. --Job xxxiii.
                                                  18.
      [1913 Webster]

   3. A covered deep hole for entrapping wild beasts; a pitfall;
      hence, a trap; a snare. Also used figuratively.
      [1913 Webster]

            The anointed of the Lord was taken in their pits.
                                                  --Lam. iv. 20.
      [1913 Webster]

   4. A depression or hollow in the surface of the human body;
      as:
      (a) The hollow place under the shoulder or arm; the
          axilla, or armpit.
      (b) See Pit of the stomach (below).
      (c) The indentation or mark left by a pustule, as in
          smallpox.
          [1913 Webster]

   5. Formerly, that part of a theater, on the floor of the
      house, below the level of the stage and behind the
      orchestra; now, in England, commonly the part behind the
      stalls; in the United States, the parquet; also, the
      occupants of such a part of a theater.
      [1913 Webster]

   6. An inclosed area into which gamecocks, dogs, and other
      animals are brought to fight, or where dogs are trained to
      kill rats. "As fiercely as two gamecocks in the pit."
      --Locke.
      [1913 Webster]

   7. [Cf. D. pit, akin to E. pith.] (Bot.)
      (a) The endocarp of a drupe, and its contained seed or
          seeds; a stone; as, a peach pit; a cherry pit, etc.
      (b) A depression or thin spot in the wall of a duct.
          [1913 Webster]

   Cold pit (Hort.), an excavation in the earth, lined with
      masonry or boards, and covered with glass, but not
      artificially heated, -- used in winter for the storing and
      protection of half-hardly plants, and sometimes in the
      spring as a forcing bed.

   Pit coal, coal dug from the earth; mineral coal.

   Pit frame, the framework over the shaft of a coal mine.

   Pit head, the surface of the ground at the mouth of a pit
      or mine.

   Pit kiln, an oven for coking coal.

   Pit martin (Zool.), the bank swallow. [Prov. Eng.]

   Pit of the stomach (Anat.), the depression on the middle
      line of the epigastric region of the abdomen at the lower
      end of the sternum; the infrasternal depression.

   Pit saw (Mech.), a saw worked by two men, one of whom
      stands on the log and the other beneath it. The place of
      the latter is often in a pit, whence the name.

   pit stop, See pit stop in the vocabulary.

   Pit viper (Zool.), any viperine snake having a deep pit on
      each side of the snout. The rattlesnake and copperhead are
      examples.

   Working pit (Min.), a shaft in which the ore is hoisted and
      the workmen carried; -- in distinction from a shaft used
      for the pumps.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pit stop
    n 1: a stop during an automobile trip for rest and refreshment
    2: a brief stop at a pit during an automobile race to take on
       fuel or service the car