What is Certainty?

Legal Definition
Certainty is perfect knowledge that has total security from error, or the mental state of being without doubt.

Objectively defined, certainty is total continuity and validity of all foundational inquiry, to the highest degree of precision. Something is certain only if no skepticism can occur. Philosophy (at least, historical Cartesian philosophy) seeks this state.

It is widely held that certainty about the real world is a failed historical enterprise (that is, beyond deductive truths, tautology, etc.). This is in large part due to the power of David Hume's problem of induction. Physicist Carlo Rovelli adds that certainty, in real life, is useless or often damaging (the idea is that "total security from error" is impossible in practice, and a complete "lack of doubt" is undesirable).
-- Wikipedia
Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
In pleading. Distinctness ; clearness of statement; particularity. Such precision and explicitness in the statement of alleged facts that the pleader's averments and contention may be readily understood by the plcader on the other side, as well as by the court and jury. State v. Hayward, 83 Mo. 309; State v. Burke, 151 Mo. 143, 52 S. W. 226; David v. David, 66 Ala. 148. This word is technically used in pleading in two different senses, signifying either distinctness, or particularity, as opposed to undue generality. Certainty is said to be of three sorts:

(1) Certainty to a common intent is such as is attained by using words in their ordinary meaning, but is not exclusive of another meaning which might be made out by argument or inference.

(2) Certainty to a certain intent in general is that which allows of no misunderstanding if a fair and reasonable construction is put upon the language employed, without bringing in facts which are possible, but not apparent.

(3) Certainty to a certain intent in particular is the highest degree of technical accuracy and precision. Co. Litt. 303 ; 2 H. Bl. 530; Spencor v. Southwick, 9 Johns. (N. Y.) 317; State v. Parker, 34 Ark. 158, 36 Am. Rep. 5. In contracts. The quality of being specific, acourate, and distinct A thing is certain when its essence, quality, and quantity are described, distinctly set forth, etc. Dig. 12, 1, 6. It is uncertain when the description is not that of an individual object, but designates only the kind. Civ. Code La. art. 3522, no. 8; 5 Coke, 121.
-- Black's Law Dictionary
Legal Definition
Distinctness and accuracy of statement.—Bouvier Law Dictionary.
-- Ballentine's Law Dictionary