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transitive verb.
1. to make different; change the appearance of; vary.
△Ex. If this coat is too large, a tailor can alter it to fit you.
(SYN) modify.
2. (U.S. and Australia.)
a. to castrate (a male animal).
b. to spay (a female animal).
v.i. to become different.
△Ex. Since her trip to Europe, her whole outlook has altered.