alteryoudaoicibaDictYouDict[alter 词源字典]
alter: [14] Alter comes from the Latin word for ‘other (of two)’, alter. In late Latin a verb was derived from this, alterāre, which English acquired via French altérer. Latin alter (which also gave French autre and English alternate [16], alternative [17], altercation [14], and altruism, not to mention alter ego) was formed from the root *al- (source of Latin alius – from which English gets alien, alias, and alibi – Greek allos ‘other’, and English else) and the comparative suffix *-tero-, which occurs also in English other.

Hence the underlying meaning of Latin alter (and, incidentally, of English other) is ‘more other’, with the implication of alternation between the two.
=> alias, alien, alternative, altruism, else[alter etymology, alter origin, 英语词源]
alter (v.)youdaoicibaDictYouDict
late 14c., "to change (something)," from Old French alterer "change, alter," from Medieval Latin alterare "to change," from Latin alter "the other (of the two)," from PIE *al- "beyond" (see alias (adv.)) + comparative suffix -ter (as in other). Intransitive sense "to become otherwise" first recorded 1580s. Related: Altered; altering.