The Collaborative International Dictionary of English v.0.48:

Certainty \Cer"tain*ty\, n.; pl. Certainties. [OF.
   certainet['e].]
   1. The quality, state, or condition, of being certain.
      [1913 Webster]

            The certainty of punishment is the truest security
            against crimes. --Fisher Ames.
      [1913 Webster]

   2. A fact or truth unquestionable established.
      [1913 Webster]

            Certainties are uninteresting and sating. --Landor.
      [1913 Webster]

   3. (Law) Clearness; freedom from ambiguity; lucidity.
      [1913 Webster]

   Of a certainty, certainly.
      [1913 Webster]

WordNet (r) 3.0 (2006):

certainty
    n 1: the state of being certain; "his certainty reassured the
         others" [ant: doubt, doubtfulness, dubiety,
         dubiousness, incertitude, uncertainty]
    2: something that is certain; "his victory is a certainty" [syn:
       certainty, sure thing, foregone conclusion] [ant:
       precariousness, uncertainness, uncertainty]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "certainty":
   absolutely, absoluteness, acceptation, acception, acquiescence,
   act of God, actuality, anticipation, assurance, assuredly,
   assuredness, authoritativeness, belief, certainly, certitude,
   confidence, connection, contemplation, conviction, credence,
   credit, credulity, definitely, definiteness, dependence, dogmatism,
   expectancy, expectation, fact, faith, fate, fatefulness, firmness,
   for a certainty, for sure, force majeure, hope, imminence,
   indefeasibility, indubitably, ineluctability, inescapableness,
   inevasibleness, inevitability, inevitable accident, inevitableness,
   inexorability, inflexibility, irrevocability, necessity,
   positively, positiveness, positivism, predetermination,
   probability, prospect, reality, reception, relentlessness,
   reliance, reliance on, self-assurance, staunchness, steadiness,
   stock, store, sure thing, surely, sureness, surety,
   suspension of disbelief, thought, trust, truth, unastonishment,
   unavoidable casualty, unavoidableness, uncontrollability,
   undeflectability, undeniably, undoubtedly, unpreventability,
   unquestionably, unyieldingness, vis major



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

CERTAINTY, pleading. By certainty is understood a clear and distinct
statement of the facts which constitute the cause of action, or ground of
defence, so that they may be understood by the party who is to answer them,
by the jury who are to ascertain the truth of the allegations, and by the
court who are to give the judgment. Cowp. 682; Co. Litt. 308; 2 Bos. & Pull.
267; 13 East, R. 107; Com. Dig. Pleader, C 17; Hob. 295. Certainty has been
stated by Lord Coke, Co. Litt. 303, a, to be of three sorts namely, 1.
certainty to a common intent 2. to a certain intent in general; and, 3. to a
certain intent in every particular. In the case of Dovaston.v. Paine Buller,
J. said he remembered to have heard Mr. Justice Ashton treat these
distinctions as a jargon of words without meaning; 2 H. Bl. 530. They have,
however, long been made, and ought not altogether to be departed from.
     2.-1. Certainty to a common intent is simply a rule of construction.
It occurs when words are used which will bear a natural sense, and also an
artificial one, or one to be made out by argument or inference. Upon the
ground of this rule the natural sense of words is adopted, without addition.
2 H. Bl. 530.
     3.-2. Certainty to, a certain intent in general, is a greater degree
of certainty than the last, and means what upon a fair and reasonable
construction may be called certain, without recurring to possible facts
which do not appear; 9 Johns. R. 317; and is what is required in
declarations, replications, and indictments, in the charge or accusation,
and in returns to writs of mandamus. See 1 Saund. 49, n. 1; 1 Dougl. 159; 2
Johns. Cas. 339; Cowp. 682; 2 Mass. R. 363 by some of which authorities, it
would seem, certainty to a common intent is sufficient in a declaration.
     4.-3. The third degree of certainty, is that which precludes all
argument, inference, or presumption against the party, pleading, and is that
technical accuracy which is not liable to the most subtle and scrupulous
objections, so that it is not merely a rule of construction, but of
addition; for where this certainty is necessary, the party must not only
state the facts of his case in the most precise way, but add to them such as
show that they are not to be controverted, and, as it were, anticipate the
case of his adversary. Lawes on Pl. 54, 55. See 1 Chitty on Pl. 235 to 241.



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

CERTAINTY, UNCERTAINTY, contracts. In matters of obligation, a thing is
certain, when its essence, quality, and quantity, are described, distinctly
set forth, Dig. 12, 1, 6. It is uncertain, when the description is not that
of one individual object, but designates only the kind. Louis. Code, art.
3522, No. 8 5 Co. 121. Certainty is the mother of repose, and therefore the
law aims at certainty. 1 Dick. 245. Act of the 27th of July, 1789, ii. 2, 1
Story's Laws, 6. His compensation for his servicer, shall not exceed two
thousand dollars per annum. Gordon's Dig. art. 211.
     2. If a contract be so vague in its terms, that its meaning cannot be
certainly collected, and the statute of frauds preclude the admissibility of
parol evidence to clear up the difficulty; 5 Barn. & Cr. 588; S. C. 12 Eng.
Com. L. R. 827; or parol evidence cannot supply the defect, then neither at
law, nor in equity, can effect be given to it. 1 Russ. & M. 116; 1 Ch. Pr.
123.
     3. It is a maxim of law, that, that is certain which may be made
certain; certum est quod certum reddi potest Co. Litt. 43; for example, when
a man sells the oil he has in his store at so much a gallon, although there
is uncertainty as to the quantity of oil, yet inasmuch as it can be
ascertained, the maxim applies, and the sale is good. Vide generally, Story,
Eq. El. Sec. 240 to 256; Mitf. Pl. by Jeremy, 41; Coop. Eq. Pl. 5; Wigr. on
Disc. 77.