The Collaborative International Dictionary of English v.0.48:

Tenacious \Te*na"cious\, a. [L. tenax, -acis, from tenere to
   hold. See Tenable, and cf. Tenace.]
   1. Holding fast, or inclined to hold fast; inclined to retain
      what is in possession; as, men tenacious of their just
      rights.
      [1913 Webster]

   2. Apt to retain; retentive; as, a tenacious memory.
      [1913 Webster]

   3. Having parts apt to adhere to each other; cohesive; tough;
      as, steel is a tenacious metal; tar is more tenacious than
      oil. --Sir I. Newton.
      [1913 Webster]

   4. Apt to adhere to another substance; glutinous; viscous;
      sticking; adhesive. "Female feet, too weak to struggle
      with tenacious clay." --Cowper.
      [1913 Webster]

   5. Niggardly; closefisted; miserly. --Ainsworth.
      [1913 Webster]

   6. Holding stoutly to one's opinion or purpose; obstinate;
      stubborn.
      [1913 Webster] -- Te*na"cious*ly, adv. --
      Te*na"cious*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tenacious
    adj 1: good at remembering; "a retentive mind"; "tenacious
           memory" [syn: retentive, recollective, long,
           tenacious] [ant: forgetful, short, unretentive]
    2: stubbornly unyielding; "dogged persistence"; "dour
       determination"; "the most vocal and pertinacious of all the
       critics"; "a mind not gifted to discover truth but tenacious
       to hold it"- T.S.Eliot; "men tenacious of opinion" [syn:
       dogged, dour, persistent, pertinacious, tenacious,
       unyielding]
    3: sticking together; "two coherent sheets"; "tenacious burrs"
       [syn: coherent, tenacious]

Moby Thesaurus II by Grady Ward, 1.0:

222 Moby Thesaurus words for "tenacious":
   adamant, adherent, adhesive, amylaceous, assiduous, balking, balky,
   bigoted, bound, bound and determined, bulldogged, bulldoggish,
   bulldoggy, bulletheaded, bullheaded, cartilaginous, case-hardened,
   chewy, clabbered, clammy, clinging, clingy, clotted, coagulated,
   cohesive, committed, constant, continuing, coriaceous, curdled,
   decided, decisive, dedicated, definite, determined, devoted,
   diligent, dogged, dogmatic, doughy, durable, earnest, enduring,
   faithful, fanatic, fibrous, firm, fixed, game, gamy, gaumy,
   gelatinous, glairy, gluelike, gluey, glutenous, glutinose,
   glutinous, good, gooey, grasping, gripping, gristly, gritty,
   grumous, gumbo, gumbolike, gumlike, gummous, gummy, gutsy, gutty,
   hard, hardheaded, hardy, headstrong, heavy, holding, immovable,
   immutable, inalterable, indefatigable, indomitable, industrious,
   inflexible, insistent, inspissated, intolerant, intransigent,
   invincible, jelled, jellied, jellylike, keeping, lasting,
   leatherlike, leathery, loyal, maintaining, mettlesome,
   mucilaginous, mulish, nervy, never-tiring, obdurate, obstinate,
   opinionated, overzealous, pasty, patient, patient as Job,
   permanent, perseverant, persevering, persistent, persisting,
   pertinacious, pigheaded, plodding, plucky, plugging, preoccupied,
   purposeful, rapt, red-blooded, refractory, relentless, resistant,
   resolute, resolved, restive, retaining, retentive, rigid, ropy,
   secure, sedulous, self-adhesive, self-willed, serious, set,
   sincere, sinewy, single-minded, slabby, sleepless, slimy, slithery,
   slogging, spirited, spiritful, spunky, stable, stalwart, starchy,
   staunch, steadfast, steady, stickable, sticky, stiff, stiff-necked,
   stodgy, stout, stringy, strong, strong-minded, strong-willed,
   strongheaded, stubborn, sturdy, sulky, sullen, syrupy, tacky,
   thick, thickened, tight, tireless, tough, tough as leather,
   tremelloid, tremellose, true, unabating, uncompromising,
   unconquerable, uncooperative, undaunted, undiscouraged, undrooping,
   unfailing, unfaltering, unflagging, unflinching, unintermitting,
   uninterrupted, unnodding, unregenerate, unrelaxing, unrelenting,
   unremitting, unshakable, unshakeable, unshaken, unsleeping,
   unswerving, untiring, unwavering, unwearied, unwearying, unwinking,
   unyielding, utterly attentive, vigorous, viscid, viscose, viscous,
   weariless, wholehearted, willful, wiry