What is Alter?

Legal Definition
To change. Alterations are made either in the contract itself, or in the instrument which is evidence of it. The contract may at any time be altered with the consent of the parties, and the alteration may be either in writing or not in writing.

2. It is a general rule that the terms of a contract under seal, cannot be changed by a parol agreement. Cooke, 500; 3 Blackf. R. 353; 4 Bibb. 1. But it has been decided that an alteration of a contract by specialty, made by parol, makes it all parol. 2 Watts, 451; 1 Wash. R. 170; 4 Cowen, 564; 3 Harr. & John. 438; 9 Pick. 298; 1 East, R. 619; but see 3 S.& R. 579.

3. When the contract is, in writing, but not under seal, it may be varied by parol, and the whole will make but one agreement. 9 Cowen, 115; 5.N. H. Rep. 99; 6 Harr. & John, 38; 18 John. 420; 1 John. Cas. 22; 5 Cowen, 606; Pet. C. C. R. 221; 1 Fairf. 414.

4. When the contract is evidenced by a specialty, and it is altered by parol, the whole will be considered as a parol agreement. 2 Watt 451; 9 Pick. 298. For alteration of instruments see Erasure; Interlineation. See, generally, 7 Greenl. 76, 121, 394; 15 John. 200; 2 Penna. R. 454.
-- Bouviers Law Dictionary
Legal Definition
To make a change in; to modify ; to vary in some degree; to change some of the elements or ingredients or details without substituting an entirely new thing or destroying the identity of the thing affected. Hannibal v. Winchell. 54 Mo. 177; Haynes v. State, 15 Ohio St. 455; Davis v. Campbell, 93 Iowa, 524, 61 N. W. 1053; Sessions v. State, 115 Ga. 18, 41 S. E. 259. See ANTERATION. Synonyms. This term is to be distinguished from its synonyms "change" and "amend." To change may import the substitution of an entirely different thing, while to alter is to operate upon a subject-matter which continues objectively the same while modified in some particular. If a check is raised, in respect to its amount, it is altered; if a new check is put in its place, it is changed. To "amend" implies that the modification made in the subject improves it, which is not necessarily the case with an alteration. An amendment always involves an alteration, but an alteration does not always amend.
-- Black's Law Dictionary