The Collaborative International Dictionary of English v.0.48:

Occur \Oc*cur"\, v. i. [imp. & p. p. Occurred; p. pr. & vb. n.
   Occurring.] [L. occurrere, occursum; ob (see Ob-) +
   currere to run. See Course.]
   1. To meet; to clash. [Obs.]
      [1913 Webster]

            The resistance of the bodies they occur with.
                                                  --Bentley.
      [1913 Webster]

   2. To go in order to meet; to make reply. [Obs.]
      [1913 Webster]

            I must occur to one specious objection. --Bentley.
      [1913 Webster]

   3. To meet one's eye; to be found or met with; to present
      itself; to appear.
      [1913 Webster]

            In Scripture, though the word heir occur, yet there
            is no such thing as "heir" in our author's sense.
                                                  --Locke.
      [1913 Webster]

   4. To happen; to take place; as, I will write if opportunity
      occurs.
      [1913 Webster]

   5. To come to the mind; to arise in the mind; to suggest
      itself; to be presented to the imagination or memory; --
      of an idea or thought; as, it never occurred to me to call
      John to ask.
      [1913 Webster +PJC]

            There doth not occur to me any use of this
            experiment for profit.                --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

occur
    v 1: come to pass; "What is happening?"; "The meeting took place
         off without an incidence"; "Nothing occurred that seemed
         important" [syn: happen, hap, go on, pass off,
         occur, pass, fall out, come about, take place]
    2: come to one's mind; suggest itself; "It occurred to me that
       we should hire another secretary"; "A great idea then came to
       her" [syn: occur, come]
    3: to be found to exist; "sexism occurs in many workplaces";
       "precious stones occur in a large area in Brazil"

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "occur":
   appear, arise, be, be extant, be found, be in existence,
   be located, be met with, be present, be realized, be situated,
   be the case, be there, become manifest, befall, betide, breathe,
   chance, come, come about, come down, come off, come to,
   come to mind, come to pass, come true, crop up, dawn on,
   dawn upon one, develop, dwell in, eventuate, exist, fall, go off,
   grab one, hap, happen, happen to be, have being, have place, hit,
   hold, indwell, inhere, lie, live, manifest itself, materialize,
   obtain, occur to, offer itself, pass, pass off, present itself,
   prevail, remain, rise to mind, stand, strike, strike the mind,
   subsist, suggest itself, surface, take place, transpire, turn up