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.
      [1913 Webster]

   2. A stain on character or reputation; something that soils
      purity; disgrace; reproach; fault; blemish.
      [1913 Webster]

            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.
      [1913 Webster]

   4. A small extent of space; a place; any particular place.
      "Fixed to one spot." --Otway.
      [1913 Webster]

            That spot to which I point is Paradise. --Milton.
      [1913 Webster]

            "A jolly place," said he, "in times of old!
            But something ails it now: the spot is cursed."
                                                  --Wordsworth.
      [1913 Webster]

   5. (Zool.) A variety of the common domestic pigeon, so called
      from a spot on its head just above its beak.
      [1913 Webster]

   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]
      [1913 Webster]

   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"

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "on the spot":
   PDQ, aboard, among us, apace, as of now, as things are, at once,
   at present, at this juncture, at this moment, at this point,
   at this time, between two fires, but now, cornered, cursory,
   decisively, directly, endangered, even now, expeditious,
   expeditiously, festinate, feverish, flying, for the nonce,
   for this occasion, forthwith, furious, hasty, here, here and now,
   hereabout, hereabouts, hereat, hereinto, hereto, hereunto,
   hic et nunc, hither, hitherward, hitherwards, hurried, immediate,
   immediately, imperiled, in Dutch, in a bind, in a fix, in a mess,
   in a pickle, in a predicament, in a scrape, in a spot, in danger,
   in deep water, in desperate case, in extremis, in jeopardy,
   in no time, in our time, in these days, in this place,
   in this vicinity, in trouble, instant, instantaneously, instanter,
   instantly, jeopardized, just here, just now, just then,
   last-minute, momentaneously, momentarily, momently, now, nowadays,
   on Queer Street, on board, on the dot, on the instant, on the nail,
   passing, pretty damned quick, prompt, promptly, pronto, quick,
   quickly, right away, right off, slap-bang, slapdash, smartly, snap,
   somewhere about, speedily, speedy, straightaway, straightway,
   subito, summarily, superficial, swift, swiftly, this day,
   this night, threatened, to this place, today, tonight, urgent,
   with all speed, with us, without delay, without further delay