c.1374, "to change (something)," from O.Fr.
altérer, from M.L.
alterare, from L.
alter "the other (of the two)," from PIE
*al- "beyond" + comp. suffix
-ter (cf.
other). Intr. sense "to become otherwise" first recorded 1590.
Alteration "change in ready-made clothes to suit a customer's specifications" is from 1901.