What is Alteration?

Legal Definition
An act done upon an instrument in writing by a party entitled under it, without the consent of the other party, by which its meaning or language is changed; it imports some fraud or design on the part of him who made it. This differs from spoliation, which is the mutilation of the instrument by the act of a stranger.

2. When an alteration has a tendency to mislead, by so changing the character of the instrument, it renders it void; but if the change has not such tendency, it will not be considered an alteration. 1 Greenl. Ev. 566.

3. A spoliation, on the contrary, will not affect the legal character of the instrument, so long as the original writing remains legible; and, if it be a deed, any trace of the seal remains. 1 Greenl. Ev. 566. See Spoliation.
-- Bouviers Law Dictionary
Legal Definition
Variation; changing; making different. See Alter.

An alteration is an act done upon the instrument by which its meaning or language is changed. If what is written upon or erased from the instrument has no tendency to produce this result, or to mislead any person, it is not an alteration. Oliver v. Hawley, 5 Neb. 444.

An alteration is said to be material when it affects, or may possibly affect, the rights of the persons interested in the document.

Synonyms. An act done upon a written instrument, which, without destroying the identity of the document, introduces some change into its terms, meaning, language, or details is an alteration. This may be done either by the mutual agreement of the parties concerned, or by a person interested under the writing without the consent, or without the knowledge, of the others. In either case it is properly denominated an alteration; but if performed by a mere stranger, it is more technically described as a spoliation or mutilation. Co.'chran v. Ne-beker, 48 Ind. 462.

The term is not properly applied to any change which involves the substitution of a practically new document. And it should in strictness be reserved for the designation of changes in form or language, and not used with reference to modifications in matters of substance. The term is also to be distinguished from "defacement," which conveys the idea of an obliteration or destruction of marks, signs, or characters already existing. An addition which does not change or interfere with the existing marks or signs, but gives a different tenor or significance to the whole, may be an alteration, but is not a defacement. Lmney v. State, 6 Tex. 1, 55 Am. Dec. 756.

Again, in the law of wills, there is a difference between revocation and alteration. If what is done simply takes away what was given before, or a part of it, it is a revocation ; but if it gives something in addition or in substitution, then it is an alteration. Appeal of Miles, 68 Co.nn. 237, 36 Atl. 39, 36 L. R. A. 176.
-- Black's Law Dictionary
Legal Definition
Any changes done to a building in the remodeling process. AKA alterations and extensions.
Legal Definition
A change in a document by which its legal effect is altered. See 148 111. 349, 35 N. E. 1120.
-- Ballentine's Law Dictionary