The Collaborative International Dictionary of English v.0.48:

Certain \Cer"tain\, a. [F. certain, fr. (assumed) LL. certanus,
   fr. L. certus determined, fixed, certain, orig. p. p. of
   cernere to perceive, decide, determine; akin to Gr. ? to
   decide, separate, and to E. concern, critic, crime, riddle a
   sieve, rinse, v.]
   1. Assured in mind; having no doubts; free from suspicions
      concerning.
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            To make her certain of the sad event. --Dryden.
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            I myself am certain of you.           --Wyclif.
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   2. Determined; resolved; -- used with an infinitive.
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            However, I with thee have fixed my lot,
            Certain to undergo like doom.         --Milton.
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   3. Not to be doubted or denied; established as a fact.
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            The dream is certain, and the interpretation thereof
            sure.                                 --Dan. ii. 45.
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   4. Actually existing; sure to happen; inevitable.
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            Virtue that directs our ways
            Through certain dangers to uncertain praise.
                                                  --Dryden.
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            Death, as the Psalmist saith, is certain to all.
                                                  --Shak.
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   5. Unfailing; infallible.
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            I have often wished that I knew as certain a remedy
            for any other distemper.              --Mead.
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   6. Fixed or stated; regular; determinate.
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            The people go out and gather a certain rate every
            day.                                  --Ex. xvi. 4.
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   7. Not specifically named; indeterminate; indefinite; one or
      some; -- sometimes used independenty as a noun, and
      meaning certain persons.
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            It came to pass when he was in a certain city.
                                                  --Luke. v. 12.
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            About everything he wrote there was a certain
            natural grace und decorum.            --Macaulay.
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   For certain, assuredly.

   Of a certain, certainly.

   Syn: Bound; sure; true; undeniable; unquestionable;
        undoubted; plain; indubitable; indisputable;
        incontrovertible; unhesitating; undoubting; fixed;
        stated.
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The Collaborative International Dictionary of English v.0.48:

Certain \Cer"tain\, n.
   1. Certainty. [Obs.] --Gower.
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   2. A certain number or quantity. [Obs.] --Chaucer.
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The Collaborative International Dictionary of English v.0.48:

Certain \Cer"tain\, adv.
   Certainly. [Obs.] --Milton.
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WordNet (r) 3.0 (2006):

certain
    adj 1: definite but not specified or identified; "set aside a
           certain sum each week"; "to a certain degree"; "certain
           breeds do not make good pets"; "certain members have not
           paid their dues"; "a certain popular teacher"; "a certain
           Mrs. Jones"
    2: having or feeling no doubt or uncertainty; confident and
       assured; "felt certain of success"; "was sure (or certain)
       she had seen it"; "was very sure in his beliefs"; "sure of
       her friends" [syn: certain(p), sure] [ant: incertain,
       uncertain, unsure]
    3: established beyond doubt or question; definitely known; "what
       is certain is that every effect must have a cause"; "it is
       certain that they were on the bus"; "his fate is certain";
       "the date for the invasion is certain" [ant: uncertain]
    4: certain to occur; destined or inevitable; "he was certain to
       fail"; "his fate is certain"; "In this life nothing is
       certain but death and taxes"- Benjamin Franklin; "he faced
       certain death"; "sudden but sure regret"; "he is sure to win"
       [syn: certain, sure] [ant: uncertain]
    5: established irrevocably; "his fate is sealed" [syn: sealed,
       certain] [ant: uncertain, unsealed]
    6: reliable in operation or effect; "a quick and certain
       remedy"; "a sure way to distinguish the two"; "wood dust is a
       sure sign of termites" [syn: certain, sure]
    7: exercising or taking care great enough to bring assurance;
       "be certain to disconnect the iron when you are through"; "be
       sure to lock the doors" [syn: certain, sure]

Moby Thesaurus II by Grady Ward, 1.0:

286 Moby Thesaurus words for "certain":
   a, a certain, absolute, accurate, actual, adducible, admissible,
   agape, agog, all agog, an, anticipant, anticipating, anticipative,
   anticipatory, any, any one, apodictic, ascertained, assured,
   atomic, attestative, attested, attestive, authentic, authenticated,
   awaiting, based on, believing, bound, categorically true,
   certified, changeless, circumstantial, clear, clear and distinct,
   clear as day, cocksure, composite, conclusive, concrete, confident,
   confirmable, confirmed, constant, convinced, convincing,
   corroborated, covered, credible, cumulative, damning, decided,
   decisive, defined, definite, demonstrable, demonstrated,
   dependable, destined, detailed, determinate, determinative,
   determined, devout, different, distinct, distinguished, divers,
   documentary, documented, dogmatic, doubtless, eager, effectual,
   either, ensured, esoteric, especial, established, evidential,
   evidentiary, ex parte, exceptional, exclusive, expectant,
   expecting, express, extraordinary, eye-witness, factual, faithful,
   fated, fateful, fideistic, final, firm, firsthand, fixed,
   forearmed, forestalling, forewarned, founded on, gaping,
   grounded on, guaranteed, hearsay, historical, hopeful, implicit,
   impressed with, in anticipation, in expectation, in the cards,
   incontestable, incontrovertible, indefeasible, indicative,
   indisputable, individual, indivisible, indubitable, ineluctable,
   ineludible, inescapable, inevasible, inevitable, inexorable,
   infallible, inflexible, inner, insured, integral, intimate,
   invariable, irreducible, irrefutable, irresistible, irrevocable,
   lone, looking for, looking forward to, many, material, measured,
   minute, monadic, monistic, more, more than one, necessary,
   not in error, not singular, not surprised, noteworthy, numerous,
   nuncupative, objectively true, one, optimistic, overwhelming,
   particular, perfectly sure, permanent, personal, persuaded,
   pietistic, pious, pistic, plausible, plural, pluralistic,
   plurative, positive, precise, predestined, predetermined, prepared,
   presumptive, private, probative, provable, proved, quantified,
   quantitative, quantitive, quantized, ready, real, relentless,
   reliable, resistless, respective, sanguine, satisfied, secure,
   secured, set, settled, several, significant, simple, single,
   singular, sold on, sole, solid, solipsistic, solitary, some,
   special, specific, stable, standard, stated, steady, stipulated,
   substantiated, suggestive, sundry, sure, sure as death,
   sure as fate, sure-enough, surefire, symptomatic, telling,
   traditional, true, true as gospel, trustworthy, truthful,
   unalterable, unambiguous, unanalyzable, unavoidable, unchanging,
   unconfuted, uncontestable, uncontrollable, undeflectable,
   undeniable, undenied, under the impression, undisputed, undivided,
   undoubted, undoubtful, undoubting, unequivocal, unerring,
   unerroneous, unevadable, unfailing, unfallacious, unfalse, uniform,
   unique, unitary, univocal, unmistakable, unmistaken, unnamed,
   unpreventable, unquestionable, unquestioned, unrefuted,
   unspecified, unstoppable, unsurprised, unyielding, valid,
   validated, various, veracious, verifiable, verified, veritable,
   waiting, waiting for, warranted, watching for, weighty,
   well-grounded, whole, written