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.