early 15c., reduccioun, "a restoring to a former state" (a sense now obsolete), also "a conquest or subjugation" (of a people, etc.), from Old French reducion (13c., Modern French réduction) and directly from Latin reductionem (nominative reductio) "a leading back, restoration," noun of action from past-participle stem of reducere (see reduce). The meaning "diminution, a lessening" is from 1670s; chemical sense of "reversion to a simpler form" is from 1660s.
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.
<a href="https://www.etymonline.com/word/reductionism">Etymology of reductionism by etymonline</a>
Harper, D. (n.d.). Etymology of reductionism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/reductionism