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.

Again \A*gain"\ (?; 277), adv. [OE. agein, agayn, AS. ongegn,
   onge['a]n, against, again; on + ge['a]n, akin to Ger. gegewn
   against, Icel. gegn. Cf. {Gainsay}.]
   1. In return, back; as, bring us word again.

   2. Another time; once more; anew.

            If a man die, shall he live again?    --Job xiv. 14.

   3. Once repeated; -- of quantity; as, as large again, half as
      much again.

   4. In any other place. [Archaic] --Bacon.

   5. On the other hand. ``The one is my sovereign . . . the
      other again is my kinsman.'' --Shak.

   6. Moreover; besides; further.

            Again, it is of great consequence to avoid, etc.
                                                  --Hersche?.

   {Again and again}, more than once; often; repeatedly.

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

   {To and again}, to and fro. [Obs.] --De Foe.

   Note: Again was formerly used in many verbal combinations,
         as, again-witness, to witness against; again-ride, to
         ride against; again-come, to come against, to
         encounter; again-bring, to bring back, etc.

Source : WordNet®

now and again
     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 then}, {at times},
            {from time to time}]