millenarianism

1670s, "one who believes in the coming of the (Christian) millennium" (by 1550s in Latin plural form millenarii), from Latin millenarius "containing a thousand," from millenia "a thousand each," from mille "thousand" (see million). With -ian. As an adjective, "pertaining to the (Christian) millennium," from 1630s.

The apparent inconsistency in spelling (-n-, -nn-) results from the fact that millenarian, like millenary, does not contain the stem of the Latin annus year, which is present in millennium .... [Fowler]

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.