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.
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   2. Apt to retain; retentive; as, a tenacious memory.
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   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.
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   4. Apt to adhere to another substance; glutinous; viscous;
      sticking; adhesive. "Female feet, too weak to struggle
      with tenacious clay." --Cowper.
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   5. Niggardly; closefisted; miserly. --Ainsworth.
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   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]