The Collaborative International Dictionary of English v.0.48:

Caveat \Ca"ve*at\, n. [L. caved let him beware, pres. subj. of
   cavere to be on one's guard to, beware.]
   [1913 Webster]
   1. (Law) A notice given by an interested party to some
      officer not to do a certain act until the party is heard
      in opposition; as, a caveat entered in a probate court to
      stop the proving of a will or the taking out of letters of
      administration, etc. --Bouvier.
      [1913 Webster]

   2. (U. S. Patent Laws) A description of some invention,
      designed to be patented, lodged in the patent office
      before the patent right is applied for, and operating as a
      bar to the issue of letters patent to any other person,
      respecting the same invention.
      [1913 Webster]

   Note: A caveat is operative for one year only, but may be
         renewed.
         [1913 Webster]

   3. Intimation of caution; warning; protest.
      [1913 Webster]

            We think it right to enter our caveat against a
            conclusion.                           --Jeffrey.
      [1913 Webster]

   Caveat emptor [L.] (Law), let the purchaser beware, i. e.,
      let him examine the article he is buying, and act on his
      own judgment.
      [1913 Webster]

WordNet (r) 3.0 (2006):

caveat
    n 1: a warning against certain acts; "a caveat against unfair
         practices" [syn: caution, caveat]
    2: (law) a formal notice filed with a court or officer to
       suspend a proceeding until filer is given a hearing; "a
       caveat filed against the probate of a will"

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "caveat":
   admonishment, admonition, advice, advising, advocacy, alarm,
   bench warrant, briefing, capias, caution, cautioning, consultation,
   council, counsel, death warrant, determent, deterrence,
   deterrent example, direction, example, exhortation, expostulation,
   fieri facias, final notice, final warning, forewarning,
   frightening off, guidance, habere facias possessionem, hint,
   hortation, idea, injunction, instruction, interdict, intimidation,
   lesson, mandamus, mandate, mandatory injunction, mittimus,
   monition, moral, nisi prius, notice, notification, object lesson,
   opinion, parley, precept, process, prohibitory injunction,
   proposal, recommendation, remonstrance, search warrant, suggestion,
   talking out of, thought, threat, tip-off, ultimatum,
   verbum sapienti, warning, warning piece, warrant,
   warrant of arrest, warrant of attorney, writ



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CAVEAT, practice. That he beware. Caveat is the name of a notice given by a
party having an interest, to some officer, not to do an act, till the party
giving the notice shall have been heard; as, a caveat to the register of
wills, or judge of probate, not to permit a will to be proved, or not to
grant letters of administration, until the party shall have been heard. A
caveat is also frequently made to prevent a patent for inventions being
issued. 1 Bouv. Inst. 71, 534; 1 Burn's Ecc. Law, 19, 263; Bac. Abr.
Executors and Administrators, E 8; 3 Bl. Com. 246; Proctor's Pract. 68; 3
Bin. Rep. 314; 1 Siderf. 371 Poph. 133; Godolph. Orph. Leg. 258; 2 Brownl.
119; 2 Fonbl. Eq. book 4, pt. 2, c. 1, Sec. 3; Ayl. Parer. 145 Nelson's Ab.
h.t.; Dane's Ab. c. 223, a. 15, Sec. 2, and a. 8, Sec. 22. See 2 Chit. Pr.
502, note b, for a form.