The Collaborative International Dictionary of English v.0.48:

Assessor \As*sess"or\, n. [L., one who sits beside, the
   assistant of a judge, fr. assid?re. See Assession. LL., one
   who arranges of determines the taxes, fr. assid?re. See
   Assess, v., and cf. Cessor.]
   1. One appointed or elected to assist a judge or magistrate
      with his special knowledge of the subject to be decided;
      as legal assessors, nautical assessors. --Mozley & W.
      [1913 Webster]

   2. One who sits by another, as next in dignity, or as an
      assistant and adviser; an associate in office.
      [1913 Webster]

            Whence to his Son,
            The assessor of his throne, he thus began. --Milton.
      [1913 Webster]

            With his ignorance, his inclinations, and his fancy,
            as his assessors in judgment.         --I. Taylor.
      [1913 Webster]

   3. One appointed to assess persons or property for the
      purpose of taxation. --Bouvier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

assessor
    n 1: an official who evaluates property for the purpose of
         taxing it [syn: tax assessor, assessor]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "assessor":
   Internal Revenue Service, JA, amicus curiae, appraiser, assayer,
   barmaster, cartographer, chancellor, chorographer, circuit judge,
   customhouse, customs, estimator, evaluator, exciseman, farmer,
   gauger, geodesist, judge advocate, judge ordinary, jurat,
   justice in eyre, justice of assize, land surveyor, lay judge,
   legal assessor, master, measurer, meter, military judge,
   oceanographer, ombudsman, ordinary, police judge, presiding judge,
   probate judge, publican, puisne judge, recorder, revenuer,
   surveyor, tax assessor, tax collector, tax farmer, taxer, taxman,
   topographer, valuator, valuer, vice-chancellor