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(one's) master; (one's) employer; (one's) lord
「主」 has other readings with different meanings.
この古い城の主は有名な武将だった。
The lord of this old castle was a famous general.
犬は飼い主のもとへ走って行った。
The dog ran to its owner.
店の主が自ら料理を作ってくれた。
The owner of the shop cooked for us himself.