Source : Webster's Revised Unabridged Dictionary (1913)

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.

            To make her certain of the sad event. --Dryden.

            I myself am certain of you.           --Wyclif.

   2. Determined; resolved; -- used with an infinitive.

            However, I with thee have fixed my lot, Certain to
            undergo like doom.                    --Milton.

   3. Not to be doubted or denied; established as a fact.

            The dream is certain, and the interpretation thereof
            sure.                                 --Dan. ii. 45.

   4. Actually existing; sure to happen; inevitable.

            Virtue that directs our ways Through certain dangers
            to uncertain praise.                  --Dryden.

            Death, as the Psalmist saith, is certain to all.
                                                  --Shak.

   5. Unfailing; infallible.

            I have often wished that I knew as certain a remedy
            for any other distemper.              --Mead.

   6. Fixed or stated; regular; determinate.

            The people go out and gather a certain rate every
            day.                                  --Ex. xvi. 4.

   7. Not specifically named; indeterminate; indefinite; one or
      some; -- sometimes used independenty as a noun, and
      meaning certain persons.

            It came to pass when he was in a certain city.
                                                  --Luke. v. 12.

            About everything he wrote there was a certain
            natural grace und decorum.            --Macaulay.

   {For certain}, assuredly.

   {Of a certain}, certainly.

   Syn: Bound; sure; true; undeniable; unquestionable;
        undoubted; plain; indubitable; indisputable;
        incontrovertible; unhesitating; undoubting; fixed;
        stated.

Source : WordNet®

for certain
     adv : definitely or positively (`sure' is sometimes used
           informally for `surely'); "the results are surely
           encouraging"; "she certainly is a hard worker"; "it's
           going to be a good day for sure"; "they are coming, for
           certain"; "they thought he had been killed sure
           enough"; "he'll win sure as shooting"; "they sure smell
           good"; "sure he'll come" [syn: {surely}, {certainly}, {sure},
            {for sure}, {sure enough}, {sure as shooting}]