This adjective describes someone or something that is eight days old. In Philippians 3:5, Paul lists his Jewish credentials: 'circumcised on the eighth day'—meaning when he was eight days old, in precise obedience to the Law. The term emphasizes not just the timing but the exactness: Paul wasn't a Gentile convert who adopted Jewish practices later; he was covenantally included from infancy. The compound formation (eight + day) creates a precise temporal adjective found in Jewish legal contexts.