The Collaborative International Dictionary of English v.0.48:

Vocation \Vo*ca"tion\ (v[-o]*k[=a]"sh[u^]n), n. [L. vocatio a
   bidding, invitation, fr. vocare to call, fr. vox, vocis,
   voice: cf. F. vocation. See Vocal.]
   1. A call; a summons; a citation; especially, a designation
      or appointment to a particular state, business, or
      profession.
      [1913 Webster]

            What can be urged for them who not having the
            vocation of poverty to scribble, out of mere
            wantonness make themselves ridiculous? --Dryden.
      [1913 Webster]

   2. Destined or appropriate employment; calling; occupation;
      trade; business; profession.
      [1913 Webster]

            He would think his service greatly rewarded, if he
            might obtain by that means to live in the sight of
            his prince, and yet practice his own chosen
            vocation.                             --Sir. P.
                                                  Sidney.
      [1913 Webster]

   3. (Theol.) A calling by the will of God. Specifically:
      [1913 Webster]
      (a) The bestowment of God's distinguishing grace upon a
          person or nation, by which that person or nation is
          put in the way of salvation; as, the vocation of the
          Jews under the old dispensation, and of the Gentiles
          under the gospel. "The golden chain of vocation,
          election, and justification." --Jer. Taylor.
          [1913 Webster]
      (b) A call to special religious work, as to the ministry.
          [1913 Webster]

                Every member of the same [the Church], in his
                vocation and ministry.            --Bk. of Com.
                                                  Prayer.
          [1913 Webster]

WordNet (r) 3.0 (2006):

vocation
    n 1: the particular occupation for which you are trained [syn:
         career, calling, vocation]
    2: a body of people doing the same kind of work [syn:
       occupational group, vocation]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "vocation":
   ambition, apostleship, area, art, aspiration, bag, basis, business,
   call, calling, care of souls, career, career building, careerism,
   cause, consideration, craft, cup of tea, employment, field, forte,
   game, goal, ground, guiding light, guiding star, handicraft,
   holy orders, ideal, inspiration, intention, job, lifework, line,
   line of business, line of work, lodestar, long suit, main interest,
   mainspring, manner, matter, metier, mission, motive, mystery,
   number, occupation, pastorage, pastoral care, pastorate,
   pet subject, practice, priesthood, priestship, principle,
   profession, pursuit, rabbinate, racket, reason, sacred calling,
   sake, score, source, specialism, speciality, specialization,
   specialty, spring, strong point, style, technicality, the church,
   the cloth, the desk, the ministry, the pulpit, thing, trade, type,
   ulterior motive, walk, walk of life, way, weakness, work