Why is the ninth month called September?
c.1300, "to convert into another and better form," from Old French reformer "rebuild, reconstruct, recreate" (12c.), from Latin reformare "to form again, change, transform, alter," from re- "again" (see re-) + formare "to form" (see form (n.)). Intransitive sense from 1580s.
Meaning "to bring (a person) away from an evil course of life" is recorded from early 15c.; of governments, institutions, etc., from early 15c. Related: Reformed; reforming. Reformed churches (1580s) usually are Calvinist as opposed to Lutheran. Reformed Judaism (1843) is a movement initiated in Germany by Moses Mendelssohn (1729-1786). Reform school is attested from 1859.
"any proceeding which brings back a better order of things," 1660s, from reform (v.) and in some uses from French réforme. As a branch of Judaism from 1843.