The Collaborative International Dictionary of English v.0.48:

Soon \Soon\ (s[=oo]n), adv. [OE. sone, AS. s[=o]na; cf. OFries.
   s[=o]n, OS. s[=a]na, s[=a]no, OHG. s[=a]r, Goth. suns.]
   1. In a short time; shortly after any time specified or
      supposed; as, soon after sunrise. "Sooner said than done."
      --Old Proverb. "As soon as it might be." --Chaucer.
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            She finished, and the subtle fiend his lore
            Soon learned.                         --Milton.
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   2. Without the usual delay; before any time supposed; early.
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            How is it that ye are come so soon to-day? --Ex. ii.
                                                  18.
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   3. Promptly; quickly; easily.
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            Small lights are soon blown out, huge fires abide.
                                                  --Shak.
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   4. Readily; willingly; -- in this sense used with would, or
      some other word expressing will.
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            I would as soon see a river winding through woods or
            in meadows, as when it is tossed up in so many
            whimsical figures at Versailles.      --Addison.
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   As soon as, or So soon as, immediately at or after
      another event. "As soon as he came nigh unto the camp . .
      . he saw the calf, and the dancing." --Ex. xxxii. 19. See
      So . . . as, under So.

   Soon at, as soon as; or, as soon as the time referred to
      arrives. [Obs.] "I shall be sent for soon at night."
      --Shak.

   Sooner or later, at some uncertain time in the future; as,
      he will discover his mistake sooner or later.

   With the soonest, as soon as any; among the earliest; too
      soon. [Obs.] --Holland.
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The Collaborative International Dictionary of English v.0.48:

Soon \Soon\, a.
   Speedy; quick. [Obs.] --Shak.
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WordNet (r) 3.0 (2006):

soon
    adv 1: in the near future; "the doctor will soon be here"; "the
           book will appear shortly"; "she will arrive presently";
           "we should have news before long" [syn: soon,
           shortly, presently, before long]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "soon":
   after a time, after a while, afterward, afterwards, anon,
   at bottom, at once, before long, betimes, by and by, by destiny,
   directly, ere long, eventually, expeditiously, fatally, forthwith,
   gladly, happily, hastily, hopefully, immediately, imminently,
   in a jiffy, in a moment, in a second, in a while, in a wink,
   in aftertime, in due course, in due time, in good time,
   in short order, in the future, in time, instantly, later,
   lickety-split, lief, momentarily, on the double, one day, oversoon,
   posthaste, predictably, premature, presently, previous, probably,
   promptly, pronto, proximo, quick, quickly, rapidly, readily,
   right away, seasonably, shortly, sooner or later, speedily,
   straightway, swiftly, timely, tomorrow, tout de suite, ultimately,
   untimely, willingly, without delay