voluntary, of one's own initiative, of one's own accord

perpetual, permanent, everlasting

modern

secondary

a unit of volume equal to 1,200 millet grains

personal

full-fledged, qualified, up to standard / adulthood, behaving like an adult or as a member of the society / Used other than figuratively or idiomatically: see 一人, 前. ("one person's portion")

a handful / a small amount

a bath (a single instance of taking a bath)

a (single) leaf / (figuratively) a (single) small boat / a (single) sheet (something thin: page, etc.)