Source : Webster's Revised Unabridged Dictionary (1913)

Abreast \A*breast"\, adv. [Pref. a- + breast.]
   1. Side by side, with breasts in a line; as, ``Two men could
      hardly walk abreast.'' --Macaulay.

   2. (Naut.) Side by side; also, opposite; over against; on a
      line with the vessel's beam; -- with of.

   3. Up to a certain level or line; equally advanced; as, to
      keep abreast of [or with] the present state of science.

   4. At the same time; simultaneously. [Obs.]

            Abreast therewith began a convocation. --Fuller.

Source : WordNet®

abreast
     adv : alongside each other, facing on the same direction