The Collaborative International Dictionary of English v.0.48:

Certify \Cer"ti*fy\, v. t. [imp. & p. p. Certified; p. pr. &
   vb. n. Certifying.] [F. certifier, LL. certificare; L.
   certus certain + facere to make. See Certain, and cf.
   Certificate, v. t.]
   1. To give cetain information to; to assure; to make certain.
      [1913 Webster]

            We certify the king, that . . . thou shalt have no
            portion on this side the river.       --Ezra iv. 16.
      [1913 Webster]

   2. To give certain information of; to make certain, as a
      fact; to verify. --Hammond.
      [1913 Webster]

            The industry of science at once certifies and
            greatly extends our knowledge of the vastness of the
            creation.                             --I. Taylor.
      [1913 Webster]

   3. To testify to in writing; to make a declaration
      concerning, in writing, under hand, or hand and seal.
      [1913 Webster]

            The judges shall certify their opinion to the
            chancellor, and upon such certificate the decree is
            usually founded.                      --Blackstone.
      [1913 Webster]

   Certified check, A bank check, the validity of which is
      certified by the bank on which it is drawn.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

certified \certified\ adj.
   1. endorsed authoritatively as having met certain
      requirements; guranteed; as, certified milk; certified
      mail; a certified check. Opposite of uncertified.
      [Narrower terms: certificated, credentialed, documented;
      guaranteed] See also certifiable.
      [WordNet 1.5]

   2. legally insane according to clinical criteria.

   Syn: certifiable.
        [WordNet 1.5 +PJC]

   3. holding appropriate documentation and officially on record
      as qualified to perform a specified function or practice a
      specified skill.

   Syn: qualified.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

certified
    adj 1: endorsed authoritatively as having met certain
           requirements; "a certified public accountant" [ant:
           uncertified]
    2: fit to be certified as insane (and treated accordingly) [syn:
       certifiable, certified]
    3: holding appropriate documentation and officially on record as
       qualified to perform a specified function or practice a
       specified skill; "a registered pharmacist"; "a registered
       hospital" [syn: certified, qualified]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "certified":
   accepted, acknowledged, actual, admitted, affirmed, alleged,
   allowed, announced, approved, ascertained, asserted, asseverated,
   assured, attested, authenticated, averred, avouched, avowed,
   borne out, categorically true, certain, circumstantiated, conceded,
   confessed, confirmed, corroborated, countersigned, covered,
   decided, declared, demonstrated, deposed, determinate, determined,
   documentary, effectual, endorsed, ensured, enunciated, established,
   factual, fixed, granted, guaranteed, historical, in the bag,
   insured, made sure, manifestoed, nailed down, not in error,
   notarized, objectively true, on ice, open-and-shut, pledged,
   predicated, professed, pronounced, proved, proven, ratified, real,
   received, recognized, sealed, secure, secured, set, settled, shown,
   signed, stamped, stated, substantiated, sure, sure-enough, sworn,
   sworn and affirmed, sworn to, tested, tried, true, true as gospel,
   truthful, unconfuted, undenied, underwritten, undoubted,
   unerroneous, unfallacious, unfalse, unmistaken, unquestionable,
   unrefuted, validated, veracious, verified, veritable, vouched,
   vouched for, vowed, warranted