adverb taking the 'to' particle, taru-adjective
unanimously; as one; with one accord
Literary adverb describing people coming together in complete agreement or acting in perfect unison. Often used with と. Rare in modern Japanese; found mainly in classical or formal written contexts.
The congregation unanimously agreed.
Everyone followed his proposal as one.
The exact derivation is uncertain; the kanji 翕 carries a sense of 'gathering' or 'uniting', and 然 is a suffix forming adverbs/adjectives, together conveying 'in a unified manner'.