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]