taru-adjective, adverb taking the 'to' particle, adverb
all; each and every one
Literary or formal expression meaning 'all without exception'. Often used in written language or set phrases.
参加者比比として賛成した。
Every single participant agreed.
比比たる反対意見はなかった。
There was no opposition whatsoever.
皆 is the common everyday word for 'everyone' or 'all'. 比比 is a much more formal, literary equivalent.
全て is the standard word for 'all' or 'everything'. 比比 is rarer and more emphatic, often used in formal writing.
Reduplication of 比 (hi, 'compare'), literally 'compared to compared', emphasizing that each and every one is included. The exact historical derivation is uncertain, but the form is used in classical and formal modern Japanese.