The most obvious English translation of “amortización” and “amortizar” is the cognate “amortisation” and “amortise”, but this is valid only in certain cases.
In accounting terminology, “amortisation” is the correct translation of “amortización” when referring to intangible assets, meaning the allocation of the intangible assets’ purchase value over the duration of their useful lives. With respect to tangible assets (also known as property, plant and equipment), the correct translation of “amortizar/amortización” is “depreciate/depreciation”, as in the sentence “Tangible assets are depreciated over their useful lives on a straight line basis”.
“Amortizar” can also mean “repay”, in the sense of “amortizar un préstamo”, “repay a loan”. In English, “amortise” can be used with this meaning as well, although it is not very common.