The Collaborative International Dictionary of English v.0.48:

Preoccupation \Pre*oc`cu*pa"tion\, n. [L. praeoccupatio: cf. F.
   pr['e]occupation.]
   1. The act of preoccupying, or taking possession of
      beforehand; the state of being preoccupied; prepossession.
      [1913 Webster]

   2. Anticipation of objections. [R.] --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

preoccupation
    n 1: an idea that preoccupies the mind and holds the attention
    2: the mental state of being preoccupied by something [syn:
       preoccupation, preoccupancy, absorption, engrossment]
    3: the act of taking occupancy before someone else does [syn:
       preoccupancy, preoccupation]

Moby Thesaurus II by Grady Ward, 1.0:

220 Moby Thesaurus words for "preoccupation":
   Walter Mitty, absence of mind, absentmindedness,
   absorbed attention, absorption, abstractedness, abstraction,
   abulia, adoption, adverse possession, alienation, alodium, anxiety,
   anxiety equivalent, anxiety state, apathy, application,
   appropriation, arrogation, assiduity, assiduousness, assumption,
   bemusement, brown study, bulldog tenacity, burgage,
   castle-building, catatonic stupor, claim, close study,
   colonization, colony, complex, compulsion, concentration, conquest,
   constancy, contemplation, contemplativeness, daydream, daydreamer,
   daydreaming, de facto, de jure, deep study, deep thought,
   dejection, dependency, depression, depth of thought,
   derivative title, detachment, diligence, dogged perseverance,
   doggedness, dream, dreaming, elation, emotionalism, endurance,
   engagement, engrossment, enslavement, euphoria, fantasy,
   fantasying, fascination, fee fief, fee position, fee simple,
   fee simple absolute, fee simple conditional, fee simple defeasible,
   fee simple determinable, fee tail, feodum, feud, fidelity, fiefdom,
   fit of abstraction, fixation, fixed idea, folie du doute,
   frankalmoign, free socage, freehold, gavelkind, hang-up,
   having title to, hold, holding, hypercathexis, hypochondria,
   hysteria, hysterics, idee fixe, immersion, indefatigability,
   indent, indifference, industriousness, industry, insensibility,
   insistence, insistency, intentness, involvement,
   irresistible impulse, knight service, lay fee, lease, leasehold,
   legal claim, legal possession, lethargy, loyalty, mandate, mania,
   meditation, melancholia, melancholy, mental distress, monomania,
   mooning, moonraking, morbid drive, muse, musefulness, musing,
   muted ecstasy, obsession, obsessive compulsion, obstinacy,
   occupancy, occupation, original title, owning,
   pathological indecisiveness, patience, patience of Job,
   pensiveness, permanence, perseverance, persistence, persistency,
   pertinaciousness, pertinacity, pipe dream, pipe-dreaming, plodding,
   plugging, possessing, possession, preemption, preoccupancy,
   prepossession, prescription, profound thought, property,
   property rights, proprietary rights, psychalgia,
   psychomotor disturbance, rapt attention, reflectiveness,
   relentlessness, requisition, resolution, reverie, ruling passion,
   sedulity, sedulousness, seisin, single-mindedness,
   singleness of purpose, slogging, socage, speculativeness,
   squatting, stability, stamina, stargazing, staying power,
   steadfastness, steadiness, stick-to-itiveness, stubbornness,
   studiousness, study, stupor, subjugation, sublease, submersion,
   takeover, taking over, tenaciousness, tenacity, tenancy, tenantry,
   tenure, tenure in chivalry, thoughtfulness, tic, tirelessness,
   title, trance, twitching, underlease, undertenancy,
   unremittingness, unresponsiveness, unswerving attention, usucapion,
   usurpation, villein socage, villeinhold, villenage, wistfulness,
   withdrawal, woolgathering