The Collaborative International Dictionary of English v.0.48:

Datum \Da"tum\, n.; pl. Data. [L. See 2d Date.]
   1. Something given or admitted; a fact or principle granted;
      that upon which an inference or an argument is based; --
      used chiefly in the plural.
      [1913 Webster]

            Any writer, therefore, who . . . furnishes us with
            data sufficient to determine the time in which he
            wrote.                                --Priestley.
      [1913 Webster]

   2. a single piece of information; a fact; especially a piece
      of information obtained by observation or experiment; --
      used mostly in the plural.
      [PJC]

   3. pl. (Math.) The quantities or relations which are assumed
      to be given in any problem.
      [1913 Webster]

   4. (Surveying) a point, line, or level surface used as a
      reference in measuring elevations. --RHUD
      [PJC]

   Datum line (Surv.), the horizontal or base line, from which
      the heights of points are reckoned or measured, as in the
      plan of a railway, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

datum
    n 1: an item of factual information derived from measurement or
         research [syn: datum, data point]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "datum":
   absolute fact, accepted fact, account, acquaintance, actual fact,
   admitted fact, announcement, article, aspect, axiom, bald fact,
   bare fact, basis for belief, blue book, body of evidence, briefing,
   brutal fact, bulletin, case, chain of evidence, circumstance, clue,
   cold fact, communication, communique, conceded fact, corpus, count,
   data, demonstrable fact, detail, directory, dispatch,
   documentation, element, empirical fact, enlightenment,
   established fact, evidence, exhibit, experience, expertise, facet,
   fact, fact of experience, factor, facts, factual base,
   factual information, familiarity, familiarization, gen,
   general information, given fact, grounds, grounds for belief,
   guidebook, handout, hard fact, hard information, incidental,
   incidental information, indication, indisputable fact,
   inescapable fact, info, information, instance, instruction,
   intelligence, intimacy, item, item of evidence, ken, know-how,
   knowing, knowledge, light, manifestation, mark, material grounds,
   matter, matter of fact, mention, message, minor detail, minutia,
   minutiae, muniments, mute witness, naked fact, not guesswork,
   not opinion, notice, notification, particular, piece of evidence,
   plain, point, positive fact, postulate, practical knowledge,
   premises, presentation, private knowledge, privity,
   promotional material, proof, provable fact, publication, publicity,
   ratio cognoscendi, reason to believe, regard, release,
   relevant fact, report, respect, salient fact, self-evident fact,
   self-knowledge, sidelight, sign, significant fact, simple fact,
   sober fact, statement, stubborn fact, symptom, technic, technics,
   technique, the case, the dope, the goods, the know,
   the nitty-gritty, the scoop, thing, token, transmission,
   undeniable fact, well-known fact, white book, white paper, word