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(one's) master; (one's) employer; (one's) lord
leader (of a country, group, etc.); head; chief
Lord
main thing; chief part; first consideration; primary object; principal aim
「主」 has other readings with different meanings.
この店の主は料理人として有名だ。
The owner of this shop is famous as a chef.
犬というものは主人に忠実である。
Dogs are loyal to their masters.
忠実な家臣は主のために命を懸けて戦った。
The loyal retainer fought at the risk of his life for his lord.