The Collaborative International Dictionary of English v.0.48:

Tenacity \Te*nac"i*ty\, n. [L. tenacitas: cf. F. t['e]nacit['e].
   See Tenacious.]
   1. The quality or state of being tenacious; as, tenacity, or
      retentiveness, of memory; tenacity, or persistency, of
      purpose.
      [1913 Webster]

   2. That quality of bodies which keeps them from parting
      without considerable force; cohesiveness; the effect of
      attraction; -- as distinguished from brittleness,
      fragility, mobility, etc.
      [1913 Webster]

   3. That quality of bodies which makes them adhere to other
      bodies; adhesiveness; viscosity. --Holland.
      [1913 Webster]

   4. (Physics) The greatest longitudinal stress a substance can
      bear without tearing asunder, -- usually expressed with
      reference to a unit area of the cross section of the
      substance, as the number of pounds per square inch, or
      kilograms per square centimeter, necessary to produce
      rupture.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tenacity
    n 1: persistent determination [syn: doggedness,
         perseverance, persistence, persistency, tenacity,
         tenaciousness, pertinacity]

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "tenacity":
   adhesiveness, application, assiduity, assiduousness, balkiness,
   bigotry, bottling up, bulldog courage, bulldog tenacity,
   bullheadedness, clabbering, clamminess, cling, clotting,
   coagulation, cohesiveness, colloidality, commitment, concentration,
   consistency, constancy, corking up, curdling, decidedness,
   decision, decisiveness, dedication, definiteness, determinateness,
   determination, determinedness, devotion, diligence,
   dogged perseverance, doggedness, dogmatism, doughiness, durability,
   earnestness, endurance, engrossment, fanaticism, fidelity,
   firmness, fixed mind, fortitude, gameness, gaminess, gelatinity,
   gelatinousness, gluelikeness, gluiness, glutinosity, glutinousness,
   grit, grittiness, gumlikeness, gumminess, hardheadedness,
   hardihood, hardiness, hardness, headstrongness, heaviness, holding,
   holding in, incrassation, indefatigability, industriousness,
   industry, inflexibility, inflexible will, infrangibility,
   inhibition, insistence, insistency, inspissation, intolerance,
   intransigence, jellification, jellylikeness, keeping, lastingness,
   leatherlikeness, lentor, locking in, loyalty, maintenance,
   mettlesomeness, mucilaginousness, mulishness, nerviness, obduracy,
   obstinacy, obstinateness, opinionatedness, overzealousness,
   pastiness, patience, patience of Job, permanence, perseverance,
   persistence, persistency, pertinaciousness, pertinacity,
   pigheadedness, plodding, pluckiness, plugging, power, prehension,
   preoccupation, preservation, purpose, purposefulness,
   relentlessness, repression, resilience, resistance, resoluteness,
   resolution, resolve, resolvedness, restiveness, retainment,
   retention, retentiveness, retentivity, rigidity, ropiness, sand,
   sedulity, sedulousness, self-will, seriousness, sincerity,
   single-mindedness, singleness of purpose, slabbiness, sliminess,
   slogging, snugness, spissitude, spunkiness, stability, stamina,
   staunchness, staying power, steadfastness, steadiness,
   stick-to-itiveness, stickiness, stiff neck, stiff-neckedness,
   stiffness, stodginess, strength, stringiness, strongheadness,
   stubbornness, sulkiness, sullenness, suppression, syrupiness,
   tackiness, tenaciousness, thickening, thickness, tightness,
   tirelessness, total commitment, toughness, treacliness,
   unbreakability, unbreakableness, uncompromisingness,
   uncooperativeness, unregenerateness, unremittingness,
   unswerving attention, viscidity, viscosity, viscousness, vitality,
   will, willfulness



The Devil's Dictionary (1881-1906):

TENACITY, n.  A certain quality of the human hand in its relation to
the coin of the realm.  It attains its highest development in the hand
of authority and is considered a serviceable equipment for a career in
politics.  The following illustrative lines were written of a
Californian gentleman in high political preferment, who has passed to
his accounting:

    Of such tenacity his grip
    That nothing from his hand can slip.
    Well-buttered eels you may o'erwhelm
    In tubs of liquid slippery-elm
    In vain -- from his detaining pinch
    They cannot struggle half an inch!
    'Tis lucky that he so is planned
    That breath he draws not with his hand,
    For if he did, so great his greed
    He'd draw his last with eager speed.
    Nay, that were well, you say.  Not so
    He'd draw but never let it go!