The Collaborative International Dictionary of English v.0.48:

Then \Then\ ([th][e^]n), adv. [Originally the same word as than.
   See Than.]
   1. At that time (referring to a time specified, either past
      or future).
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            And the Canaanite was then in the land. --Gen. xii.
                                                  6.
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            Now I know in part; but then shall I know even as
            also I am known.                      --1 Cor. xiii.
                                                  12.
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   2. Soon afterward, or immediately; next; afterward.
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            First be reconciled to thy brother, and then come
            and offer thy gift.                   --Matt. v. 24.
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   3. At another time; later; again.
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            One while the master is not aware of what is done,
            and then in other cases it may fall out to be own
            act.                                  --L'Estrange.
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   By then.
      (a) By that time.
      (b) By the time that. [Obs.]
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                But that opinion, I trust, by then this
                following argument hath been well read, will be
                left for one of the mysteries of an indulgent
                Antichrist.                       --Milton.
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   Now and then. See under Now, adv.

   Till then, until that time; until the time mentioned.
      --Milton.
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   Note: Then is often used elliptically, like an adjective, for
         then existing; as, the then administration.
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The Collaborative International Dictionary of English v.0.48:

Then \Then\, conj.
   1. Than. [Obs.] --Spenser.
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   2. In that case; in consequence; as a consequence; therefore;
      for this reason.
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            If all this be so, then man has a natural freedom.
                                                  --Locke.
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            Now, then, be all thy weighty cares away. --Dryden.
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   Syn: Therefore.

   Usage: Then, Therefore. Both these words are used in
          reasoning; but therefore takes the lead, while then is
          rather subordinate or incidental. Therefore states
          reasons and draws inferences in form; then, to a great
          extent, takes the point as proved, and passes on to
          the general conclusion. "Therefore being justified by
          faith, we have peace with God." --Rom. v. 1. "So then
          faith cometh by hearing, and hearing by the word of
          God." --Rom. x. 17.
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WordNet (r) 3.0 (2006):

then
    adv 1: subsequently or soon afterward (often used as sentence
           connectors); "then he left"; "go left first, then right";
           "first came lightning, then thunder"; "we watched the
           late movie and then went to bed"; "and so home and to
           bed" [syn: then, so, and so, and then]
    2: in that case or as a consequence; "if he didn't take it, then
       who did?"; "keep it then if you want to"; "the case, then, is
       closed"; "you've made up your mind then?"; "then you'll be
       rich"
    3: at that time; "I was young then"; "prices were lower back
       then"; "science as it was then taught"
    adj 1: at a specific prior time; "the then president"
    n 1: that time; that moment; "we will arrive before then"; "we
         were friends from then on"

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "then":
   above, accordingly, additionally, aforetime, after, after all,
   after that, afterwards, again, all included, also, altogether,
   among other things, ancient, and all, and also, and so, anon,
   as well, at another time, at that moment, at that time, au reste,
   because of that, because of this, before, before now, beforetime,
   beside, besides, beyond, consequently, contemporaneously, earlier,
   early, else, en plus, erenow, erewhile, ergo, erst, erstwhile,
   ex post facto, extra, farther, for lagniappe, for that,
   for that cause, for that reason, for this cause, for this reason,
   for which reason, fore, former, formerly, further, furthermore,
   hence, hereat, heretofore, historically, hitherto, immemorial, in,
   in addition, in that case, in the aftermath, in the past,
   in the sequel, in times past, inter alia, into the bargain, item,
   late, later, likewise, more, moreover, next, old, olden,
   on that account, on that ground, on that occasion, on the side,
   on this account, on top of, once, onetime, only yesterday, over,
   past, plus, prehistoric, prehistorically, previous, previously,
   primeval, primitive, prior, priorly, propter hoc, quondam, recent,
   recently, similarly, simultaneously, since, sometime, subsequently,
   thence, thereafter, thereat, therefor, therefore, thereon,
   thereupon, therewith, thus, thusly, thuswise, to boot, too, when,
   whence, wherefore, wherefrom, whilom, yesterday, yet