The Collaborative International Dictionary of English v.0.48:

Abreast \A*breast"\, adv. [Pref. a- + breast.]
   1. Side by side, with breasts in a line; as, "Two men could
      hardly walk abreast." --Macaulay.
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   2. (Naut.) Side by side; also, opposite; over against; on a
      line with the vessel's beam; -- with of.
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   3. Up to a certain level or line; equally advanced; as, to
      keep abreast of [or with] the present state of science.
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   4. At the same time; simultaneously. [Obs.]
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            Abreast therewith began a convocation. --Fuller.
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WordNet (r) 3.0 (2006):

abreast
    adv 1: alongside each other, facing in the same direction
    adj 1: being up to particular standard or level especially in
           being up to date in knowledge; "kept abreast of the
           latest developments"; "constant revision keeps the book
           au courant"; "always au fait on the latest events"; "up
           on the news" [syn: abreast(p), au courant, au fait,
           up on(p)]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "abreast":
   abeam, acquainted, along by, alongside, au courant, au fait,
   beside, by, coextensively, collaterally, contemporary, conversant,
   equidistantly, in parallel, informed, nondivergently, parallelly,
   parallelwise, red-hot, side-by-side, up, up-to-date,
   up-to-the-minute, versed