Alter, v. t. [imp. & p. p. Altered; p. pr. & vb. n. Altering.]
Etymology: [F. altérer, LL. alterare, fr. L. alter other, alius other. Cf. Else, Other.]
1. To make otherwise; to change in some respect, either partially or wholly; to vary; to modify. "To alter the king's course." "To alter the condition of a man." "No power in Venice can alter a decree." Shak. It gilds all objects, but it alters none. Pope. My covenant will I not break, nor alter the thing that is gone out of my lips. Ps. lxxxix. 34.
2. To agitate; to affect mentally. [Obs.] Milton.
3. To geld. [Colloq.]
Syn. -- Change, Alter. Change is generic and the stronger term. It may express a loss of identity, or the substitution of one thing in place of another; alter commonly expresses a partial change, or a change in form or details without destroying identity.
1 other Webster entry under this spelling
Alter, v. i.
To become, in some respects, different; to vary; to change; as, the weather alters almost daily; rocks or minerals alter by exposure. "The law of the Medes and Persians, which altereth not." Dan. vi. 8.
Shades of meaning
Fernald has no entry of its own for alter; he lists it under change.
Where he writes about it:
To change is distinctively to make a thing other than it has been, in some respect at least; to exchange to put or take something else in its place; to alter is ordinarily to change partially, to make different in one or more particulars. — from change
To turn is a popular word for change in any sense short of the meaning of exchange, being often equivalent to alter, convert, transform, transmute, etc. — from change