hentaigana: any of the hiragana made obsolete in Meiji-era reforms that enforced having one hiragana for each syllable
köken: Compound of 変体 (hentai, “variant form”) + 仮名 (kana, “the hiragana or katakana syllabaries; a syllabary character”). The kana changes to gana as an instance of rendaku (連濁).