Source : Webster's Revised Unabridged Dictionary (1913)

Now \Now\, adv. [OE. nou, nu, AS. n[=u], nu; akin to D., OS., &
   OHG. nu, G. nu, nun, Icel., n[=u], Dan., Sw., & Goth. nu, L.
   nunc, Gr. ?, ?, Skr. nu, n[=u]. [root]193. Cf. {New}.]
   1. At the present time; at this moment; at the time of
      speaking; instantly; as, I will write now.

            I have a patient now living, at an advanced age, who
            discharged blood from his lungs thirty years ago.
                                                  --Arbuthnot.

   2. Very lately; not long ago.

            They that but now, for honor and for plate, Made the
            sea blush with blood, resign their hate. --Waller.

   3. At a time contemporaneous with something spoken of or
      contemplated; at a particular time referred to.

            The ship was now in the midst of the sea. --Matt.
                                                  xiv. 24.

   4. In present circumstances; things being as they are; --
      hence, used as a connective particle, to introduce an
      inference or an explanation.

            How shall any man distinguish now betwixt a parasite
            and a man of honor ?                  --L'Estrange.

            Why should he live, now nature bankrupt is ? --Shak.

            Then cried they all again, saying, Not this man, but
            Barabbas. Now, Barabbas was a robber. --John xviii.
                                                  40.

            The other great and undoing mischief which befalls
            men is, by their being misrepresented. Now, by
            calling evil good, a man is misrepresented to others
            in the way of slander.                --South.

   {Now and again}, now and then; occasionally.

   {Now and now}, again and again; repeatedly. [Obs.] --Chaucer.

   {Now and then}, at one time and another; indefinitely;
      occasionally; not often; at intervals. ``A mead here,
      there a heath, and now and then a wood.'' --Drayton.

   {Now now}, at this very instant; precisely now. [Obs.] ``Why,
      even now now, at holding up of this finger, and before the
      turning down of this.'' --J. Webster (1607).

   {Now . . . now}, alternately; at one time . . . at another
      time. ``Now high, now low, now master up, now miss.''
      --Pope.

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).

            And the Canaanite was then in the land. --Gen. xii.
                                                  6.

            Now I know in part; but then shall I know even as
            also I am known.                      --1 Cor. xiii.
                                                  12.

   2. Soon afterward, or immediately; next; afterward.

            First be reconciled to thy brother, and then come
            and offer thy gift.                   --Matt. v. 24.

   3. At another time; later; again.

            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.

   {By then}.
      (a) By that time.
      (b) By the time that. [Obs.]

                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.

   {Now and then}. See under {Now}, adv.

   {Till then}, until that time; until the time mentioned.
      --Milton.

   Note: Then is often used elliptically, like an adjective, for
         then existing; as, the then administration.

Source : WordNet®

now and then
     adv : now and then or here and there; "he was arrogant and
           occasionally callous"; "open areas are only
           occasionally interrupted by clumps of trees"; "they
           visit New York on occasion"; "now and again she would
           take her favorite book from the shelf and read to us";
           "as we drove along, the beautiful scenery now and then
           attracted his attention" [syn: {occasionally}, {on
           occasion}, {once in a while}, {now and again}, {at
           times}, {from time to time}]