"weight, measure, or instrument by which the accuracy of others is determined," late 14c., from standard (n.1) "military standard, banner," a particular use in English of this word, but the sense evolution is "somewhat obscure" [OED]. The standard weights and measures were set by royal ordinance and were known as the king's standard, so perhaps metaphoric, the royal standard coming to stand for royal authority in matters like setting weights and measures. Hence the meaning "authoritative or recognized exemplar of quality or correctness" (late 15c.). Meaning "rule, principal or means of judgment" is from 1560s. That of "definite level of attainment" is attested from 1711 (as in standard of living, 1903).