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 meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.
Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.