ADVERB (3)

1. without delay or immediately;
- Example: "we hired her on the spot"
- Example: "thought they were going to shoot us down on the spot"

2. in a difficult situation;
- Example: "that question really put him on the spot"

3. at the place in question; there;
- Example: "they were on the spot when it happened"
- Example: "it had to be decided by the man on the spot"

The Collaborative International Dictionary of English v.0.48:

Spot \Spot\ (sp[o^]t), n. [Cf. Scot. & D. spat, Dan. spette, Sw.
   spott spittle, slaver; from the root of E. spit. See Spit
   to eject from the mouth, and cf. Spatter.]
   1. A mark on a substance or body made by foreign matter; a
      blot; a place discolored.
      [1913 Webster]

            Out, damned spot! Out, I say!         --Shak.
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   2. A stain on character or reputation; something that soils
      purity; disgrace; reproach; fault; blemish.
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            Yet Chloe, sure, was formed without a spot. --Pope.
      [1913 Webster]

   3. A small part of a different color from the main part, or
      from the ground upon which it is; as, the spots of a
      leopard; the spots on a playing card.
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   4. A small extent of space; a place; any particular place.
      "Fixed to one spot." --Otway.
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            That spot to which I point is Paradise. --Milton.
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            "A jolly place," said he, "in times of old!
            But something ails it now: the spot is cursed."
                                                  --Wordsworth.
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   5. (Zool.) A variety of the common domestic pigeon, so called
      from a spot on its head just above its beak.
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   6. (Zool.)
      (a) A sciaenoid food fish (Liostomus xanthurus) of the
          Atlantic coast of the United States. It has a black
          spot behind the shoulders and fifteen oblique dark
          bars on the sides. Called also goody, Lafayette,
          masooka, and old wife.
      (b) The southern redfish, or red horse, which has a spot
          on each side at the base of the tail. See Redfish.
          [1913 Webster]

   7. pl. Commodities, as merchandise and cotton, sold for
      immediate delivery. [Broker's Cant]
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   Crescent spot (Zool.), any butterfly of the family
      Melitaeidae having crescent-shaped white spots along the
      margins of the red or brown wings.

   Spot lens (Microscopy), a condensing lens in which the
      light is confined to an annular pencil by means of a
      small, round diaphragm (the spot), and used in dark-field
      illumination; -- called also spotted lens.

   Spot rump (Zool.), the Hudsonian godwit (Limosa
      haemastica).

   Spots on the sun. (Astron.) See Sun spot, ander Sun.

   On the spot, or Upon the spot, immediately; before
      moving; without changing place; as, he made his decision
      on the spot.

            It was determined upon the spot.      --Swift.
      [1913 Webster]

   Syn: Stain; flaw; speck; blot; disgrace; reproach; fault;
        blemish; place; site; locality.
        [1913 Webster]

WordNet (r) 3.0 (2006):

on the spot
    adv 1: without delay or immediately; "we hired her on the spot";
           "thought they were going to shoot us down on the spot"
    2: in a difficult situation; "that question really put him on
       the spot"
    3: at the place in question; there; "they were on the spot when
       it happened"; "it had to be decided by the man on the spot"