1660s, "pertaining to the millennium," from stem of millennium + -al (1). Meaning "pertaining to a period of 1,000 years" is from 1807. As a noun from 1896, originally "a thousandth anniversary." From 1991 as a generational name for those born in the mid-1980s and thus coming of age around the year 2000.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
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<a href="https://www.etymonline.com/word/millennialism">Etymology of millennialism by etymonline</a>
Harper, D. (n.d.). Etymology of millennialism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/millennialism