the whole country; all the land
Archaic literary term for the entire nation or realm, originally meaning 'within the four seas'. In modern Japanese, 全国 or 国中 are used instead.
「海内無双」という言葉は、国中に並ぶ者がないほど優れていることを意味する。
The phrase 海内無双 means being peerless throughout the land.
unparalleled in the whole country; peerless throughout the land
From Classical Chinese 海内 (hǎinèi), literally 'within the four seas', referring to the entire realm. The Japanese reading かいだい is the on'yomi of the characters.